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Based on the five-year population study of red voles Clethrionomys rutilus Pallas in southern West Siberia, we analysed the distribution of two predominating species of parasites (tapeworms Hymenolepis horrida and immature instars of ticks Ixodes persulcatus) in different demographic groups of the host, and seasonal changes of their incidence in the population. We assessed primary humoral immune response of the voles (splenic antibody-forming cells) to antigenic challenge (injection of sheep erythrocytes) in respect to occurrence of these parasites. It was revealed that infection with H. horrida significantly reduced the numbers of antibody-forming cells in immature summer-born voles. In contrast, immune responses in immature and mature voles, which where parasitized by I. persulcatus at the moment of capture, were significantly higher as compared to non-infected hosts. The possible mechanisms of influence of parasites on variability of immune reactions of voles in the population under study are discussed.

A phylogenetic system of the subgenus Microtimyobia (Myobiidae: Radfordia) elaborated for the first time by means of the software HENNIG-86 is proposed. The subgenus Microtimyobia including three species groups, lemnina, hylandi and zibethicalis, was divided for a cladistic analysis into 6 operation units based mainly on a male genital shield structure. The analysis shows, that the zibethicalis group is a polyphyletic, while the hylandi and lemnina groups are monophyletic. Host-parasite associations of myobiid mite taxa with vole taxa (Arvicolinae) and some peculiarities of mite and host taxa distribution are analysed. The zibethicalis group is represented by two species, associated with the North American rodents of the genera Ondatra and Phenacomys respectively. The hylandi group are most widely distributed among Arvicolinae taxa, both in Eurasia and North America. However, R. hylandi occurs on those vole species of the genera Microtus and Pitymys, which are distributed only in central and southern parts of North America and represent descendants of the earlier migration wave of Microtus from the Eurasia. As far as R. hylandi is also found in the pleistocenus of the Yakutia, that means that its areal was wider than in recent period. The lemnina group lives on hosts of 2 subtribes of the tribe Arvicolini (Arvicolina and Clethrionomyina) and is restricted to Eurasian range, except R. lemnina. This species is also mainly distributed on Eurasian vole species, however it occurs on vole species distributed in Alaska and being decendants of the later wave of Microtus migration. As far as R. lemnina and R. hylandi are associated with representatives of different migration waves of Microtus from the Euroasia, it is suggested that mites of the hylandi group are the original myobiid fauna of the Microtus voles. The species of the lemnina group had apparently originated on voles of the subtribe Clethrionomyina and then migrated onto phylogenetically young hosts of the subtribe Arvicolina (Euroasian species of the Microtus and related genera), where they probably eliminated mites of the hylandi group from these hosts in Euroasia. The recent pattern of myobiid species distribution on vole species is a result of both a mite cospeciation with their hosts and a Shift of hosts. Five new myobiid mite species are described and distinguished by characters as follows. R. (M.) abramovi sp. n. from Phodopus roborovskii (Cricetidae) is closely related to R. (M.) triton Fain et Lukoschus, 1977. In both sexes of the new species setae cxI 1, 2 are scale-shaped, while in R. (M.) triton these setae (cxI 1, 2) are hair-like. R. (M.) stekolnikovi sp. n. from Chionomys nivalis (type host) and Ch. gud is similar to R. (M.) lemnina (Koch, 1841). Females of new species have setae ra with 2 apical processes; female tritonymphs with long whip-like setae ic4. R. (M.) lemnina females have setae ra with 3 processes; female tritonymphs with short hair-like setae ic4. R. (M.) stenocrani sp. n. from Microtus gregalis is also closely related to R. (M.) lemnina. Females of the new species have setae ra with 2 processes; female tritonymphs with whip-like setae ic3. R. (M.) lemnina females have setae ra with 3 processes, female tritonymph with short hair-like setae ic3. R. (M.) synaptomysi sp. n. from Synaptomys borealis is closely related to R. (M.) lemmus Fainet Lukoschus, 1977. Females of the new species have setae pg3 11—13 being approximately equal in length to setae pg1, l4, setae l3 27—33; female tritonymphs with setae sci 78, d1 83, l4 63, tarsus IV without setae. In R. (M.) lemmus females: setae pg3 33, two times longer than setae pg1, l4, setae l3 45; in female tritonymphs: setae sci 47—58, d1 49—63, l4 18—39, tarsus IV with 3 setae. R. (M.) myopusi sp. n. from Myopus schisticolor is closely related to R. (M.) arctica Fain et Lukoschus, 1977. Females of these species differ by idiosomal proportions only. In R. myopusi females the ratio of length to width is 1.6—1.9, setae ra with 2 processes; in female tritonymphs setae ic4 11—18, tarsus IV with 3 setae. In R. (M.) arctica female the idiosomal ratio is about 1.4—1.5, setae ra with 3 processes; in female tritonymphs setae ic4 45, tarsus IV without setae.

The results of the first using of universal primer — polimerase chain reaction (UP—PSR) for the classification of trypanosomatids from insects and plants are presented. The dendrograms of 14 trypanosomatids cultures from insects and 3 cultures from plants derived from genetic distances were obtained. Wide specificity of insect trypanosomatids is demonstrated — for example two isolates of parasites from different insects orders appeared to be more similiar than isolates from the same host insect species. The reality of genus Proteomonas is confirmed. The results demonstrate the high level of heterogeneity of the isolates referring to the genus Leptomonas which seems to be an artificial group. It is obvious that the amount of existing genera is not enough for the description of trypanosomatids biodiversity. The applied method allows to obtain the hierarchical clusters of isolates. Thus, the system of insects trypanosomatids may be discussed in terms of traditional taxonomy.

The microsporidia Helmichia aggregata Larsson, 1982 from a fat body of Endochironomus sp. larvae and H. lacustris sp. n. from a fat body of Chironomus plumosus larvae are described based on light microscopical and ultrastructural characteristics. The earliest stage observed in both species were diplocaryotic meronts, which were inclosed in parasitophorous vacuoles. The diplocaryotic meronts matured into diplocaryotic sporonts, which divided meiotically producing 8 sporoblasts and spores with single nuclei. The spores of H. aggregata were rod-shaped, lightly bent, c. 3.5—5.5 x 1—1.3 um. The short polar filament of this species, which was completely uncoiled, reached the posterior pole of the spore. The spores of H. lacustris sp. n. were rodshaped, c. 3—3.7 x 1—1.3 um. The short polar filament reached the posterior pole and had the semi-coil. The ultrastructure of spore wall and polaroplast, which had anterior lammelar and posterior spongious parts, was similar in both species. The some polaroplast differences between Helmichia species are discussed.

Different cited evidences on parasitic amoebae and amoebo-flagellates belonging to the Lobosea and Heterolobosea classes (Pages, 1987) have been reviewed. Speciall attention is paid to various degree of their adaptation to parasitic mode of life, which ranges from a parasitism on a border with commensalism to true parasitism (both facultative and obligatory ones). Besides the coprophilous and commensal species, the number of true parasites among the Lobosea and Heterolobosea classes is comparatively small. In many cases, both facultative and obligatory parasites cause the death of their hosts. Apparently this stringly pronounced pathogenicity of parasitic amoebae and amoebo-flagellates suggest a recent origin of such parasite-host systems. Pathogens of amoebic dysentery, primary amoebic meningoencephalitis and granulomatous amoebic encephalitis are specially considered. In the Russian text-books on a parasitology the information on most of them, except Entamoeba hystolytica, is either totally absent or very scare and out of date.

The muscles, nervous, and tegumentary parts of the suckers were studied. It is concluded, that the fixatory structures of the tegument of suckers are not the microtrichiae, but hooklets. An active secretion of the sucker's tegument is probably a protective mechanism against the immune reaction of host.

A new species of anoplocephalidean cestode Paranoplocephala longivaginata sp. n. from some rodents (Cletrionomys rutilus, Sciurus vulgaris) of the Western Siberia is described. The new species differs from formerly known species of the genus Paranoplocephala by a structure of copulatory organs: the length of cirrus and the length of vagina exceed the length of genital bursa more than 1.5 times; the internal surface of the vagina is covered with hair-like bristles along all its length.

Freshwater molluscs Lymnaea stagnalis and Viviparus viviparus, infected with parthenogenetic generations of echinostomatid trematodes and reared in a fresh water containing metal ions Cu²⁺, Zn²⁺, Pb²⁺ (concentrations 5, 20, 1 mkg/l respectively), demonstrate an increasing of material accumulation of Cu²⁺ and Zn²⁺ in both species. A functional accumulation of Pb is observed in L. stagnalis only. The accumulation of these elements in excessive amount causes a damage of different systems in the infected molluscs and finally leads to their death.

Examination of 2185 shrimp specimens from the Black Sea in 1993—1996 has not recovered the presence of the isopod Bopyrus squillarum Latreille, 1802, the widely distributed parasite of shripms of the genus Palemon in Atlantic. The examination of collection materials has shown the decreasing of its occurence from 11.3% in 1970 to 1.6% in 1975, and its complete disappearance in the Black Sea in 1977. The diagnosis, description, original figures and data on B. squillarum pathogenicity are given.

An analysis of a distribution of the nematode Anisakis simplex larvae in muscles of fishes with different type of the fat deposition is carried out. A correlation between amount of deposited fat in an organ or tissue and quantitative characteristics of infection with the nematode larvae is recovered.

Descriptions and differential diagnoses of three new species of myxosporidia from the genera Leptotheca and Pseudoalataspora located in gall budder of four species of redfishes of the genus Sebastes in the North Atlantica are given. An occurrence of parasites in each species of redfishes from different areas is described.

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