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The genetic structure of populations of four helminth species from moor frog Rana arvalis, in comparison with the population-genetic structure of the host, has been studied with the gel-electrophoresis method. As compared with the host, parasites are characterized by more distinct deviation from the balance of genotypic frequencies and higher level of interpopulation genetic differences. The genetic variability indices in the three of four frog helminthes examined are lower than those in the host. Moreover, these indices are lower than the average indices typical of free-living invertebrates; this fact contradicts the opinion on polyhostality of these helminthes and their wide distribution.

The genus Microsomacanthus was created by Lopez-Neyra (1942). Its type species, Taenia microsoma Creplin, 1829, was described from the common eider Somateria mollissima and remained a collective species until Fuhrmann (1913) revealed its diagnostic features on the original material. Simultaneously he described two new species of «microso-ma-Gruppe», Microsomacanthus jaegerskioeldi (Fuhrmann, 1913) and M. diorchis (Fuhrmann, 1913). Fuhrmann stressed that under the name of Hymenolepis microsoma Cohn (1901) gave a description of quite different species (a parasite of the freshwater duck Anas penelope). Much earlier Leuckart (1879) used the same name for the denomination of a certain larval form from freshwater snails, without any experimental verification. Fuhrmann (1932), in conflict with himself, synonymized Taenia microsoma Creplin, 1829 and Hymenolepis microsoma sensu Cohn, 1901 and thus caused new inadequacy. Spassky et Spasskaja (1954) confirmed the validity of the genus Microsomacanthus Lopez-Neyra, 1942 and supplemented its diagnosis with such significant criteria as the number of rostellar hooks (10) and the absence of any accessory structures in the copulatory apparatus. Intermediate hosts of M. microsoma proved to be gammarids (Belopolskaya, 1952). Nevertheless Yamaguti (1959) synonymized a larval form from pond snails (Monocercus lymnaei Villot, 1883) with Microsomacanthus microsoma, meanwhile Hymenolepis microsoma sensu Cohn, 1901 was shown to be a synonym of Microsomacanthus compressa (Linton, 1892). The life cycle of the latter species includes copepods as intermediate hosts and freshwater gastropods as storage (reservoir) ones. Schmidt (1986) and Czaplinski and Vaucher (1994) chose M. compressa as an illustrative example of the genus Microsomacanthus instead of its type species. The diagnostic features of the genus remain vague, so the list of its synonyms attained as much as 17. Fuhrmann's material on M. microsoma, as well as on the two other species of «microsoma-Gruppe», deposited in the Museum of Natural History (Geneva) was reinvestigated. The diagnostic features of the group formulated by Fuhrmann (1913) were confirmed and enlarged. Large assemblage of species corresponding to M. microsoma by morphology (small gonads, early resorption of testes, formation of eggpacket in the uterus) and biology (gammarids as intermediate hosts) is established. This group is taken as the nominotypical subgenus Microsomacanthus (Microsomacanthus). For the antithetic group which life cycle corresponds to that of M. compressa (a species with comparatively large gonads, with testes persisting till the maturation of female gonads and eggs disseminating one by one from the uterus) we erect a new taxon, Microsomacanthus (Leucartcohnacanthus subgen. nov. Microsomacanthus paracompressa (Czaplinski, 1956) is selected as its type species. Representatives of both subgenera of the genus Microsomacanthus may have rostellar hooks of the same length. Literary data on their larval forms were often based on misidentification.

Data on fish infestation by parasites in the Umbozero Lake are given. 90 species of parasites were identified including 16 species of Myxosporea, 2 Suctoria, 18 Peritricha, 12 Monogenea, 13 Cestoda, 18 Trematoda, 5 Nematoda, 2 Acanthocephala, 2 Hirudinea, and 2 species of Crustacea. Character of parasites' distribution in fishes of this lake and ecological peculiarities in the host-parasite system are demonstrated.

Results of the helminthological dissection of 312 wild birds of 83 species from five orders were considered. Diagrams reflecting these data show the state of infestation for the orders and, separately, infestation of families in the order Passeriformes. Indices of infestation for these orders and families are also given.

Features of reproductive structure of the Thominx neopulchra hemipopulation from Daubenton's bat from Zhiguli State Reserve are studied. It is established, that females and males of Th. neopulchra have different dynamics of coming in the host population. Males of the parasite invade the of Daubenton's bat population during all the year, while females infest the host only from May to November. Maturing of helminthes become slowed in winter period. Only about a half of of the host population is involved in the forming of the reproductive structure of Th. neopulchra hemipopulation.

Infestation of vertebrate animals with the cestode Taenia martis and its larvae was investigated in south-west Belarus during 2001—2008. Obligatory definitive host (common marten) and intermediate hosts (red-backed vole, yellow-necked mouse, striped field mouse, and red squirrel) of this helminth were established for the Republic of Belarus. Description and figure of the T. martis larva is given.

According to recent data, Aedes communis is a polytopic species inhabiting both zonal and intrazonal biotopes. It has a wide Holarctic nemoral-tundra-steppe range, which can be characterized as European-Asiatic-North-American. In the North-West of the European part of Russia, A. communis occurs in early spring and summer, being a monocyclic species. It is distributed everywhere, both in zones of tundra and forest-tundra and in all taiga subzones. Aedes communis is a psychrophilic species, but attacks of its females were recorded under the temperatures from 2.5 to 28°C. The species occurs in biotopes of different types, but in the taiga zone it is distributed mainly in swampy coniferous forests and, particularly, in fir-woods where its quantity is steadily high, amounting to 70% and more of the mosquito females collections. When fir-woods are cut down, A. communis develops in the reservoirs situated in the small-leaved forests forming in taiga after deforestation.

Mallophaga parasitizing wild and domestic birds in the Central Ciscaucasia were studied; 8805 chewing lice specimens were collected and identified. At present, 102 species of Mallophaga are known from this territory, including 15 species recorded in the Central Ciscaucasia for the first time. Most of the chewing lice species in the region under study are parasites of Passeriformes. Species diversity of chewing lice connected with each of other 12 bird orders is several times lesser. The exchange of chewing lice between some species of hosts, particularly between domestic and wild birds, seems possible.

It is established, that infestation with Gurleya sp. (Microsporidia, Gurleyidae) changes the behaviour of its host Moina macrocopa (Straus, 1820), a crustacean inhabiting pools. Heavily infested host individuals attach to surface film of water before their death by senescence. As a consequence, their dead bodies filled with spores of the parasite adhere to objects dipped in water that probably facilitates the transfer of spores by birds and cattle to other pools. Weakly infested or uninfested individuals drown after the end of their ontogenesis. Distribution of the microsporidian spores by two different ways (with attached dead bodies of the hosts or with bottom sediments) makes invasion of a new host more probable.