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In four extensive disjunct areas of the distribution range of Ixodes crenulatus Koch, 1844 complexes of samples in 8 locations, and separate samples in two locations have been studied (fig.). Morphological characters (sizes of organs of idiosoma, gnathosoma, legs, as well as some proportions of organs), which show statistically significant differences between complexes of samples on all corresponding stages of ontogenesis were revealed (tabl. 1, 2). Statistically significant differences were determined by Student's criterion (tabl. 3, 4). We use the term «transit» characters to denote these characters if they have to addition a similar tendency at all stages of ontogenesis. Complexes of samples showing statistically significant differences of transit characters are considered by us as morphotypes. Apparently these differences of morphotypes were formed evolutionally. In European disjunct area morphotypes divided into two groups on the basis of the degree of differences: western (A, B), and eastern (C, D, E). Degree of differences of morphotypes within each group is low, whereas one between of these two groups is high and corresponds to differences of morphotypes A, В from all Asian morphotypes (F, G, H, I, J), and of morphotypes C, D, E from Asian ones in mountain disjunct areas I. crenulatus (G, H, I, J). At the same time Eastern-European complex of morphotypes (C, D, E) is morphologically similar to the morphotype from North-Kazakhstan disjunct area (F). All Asian morphotypes have high degree of differences one from the other. Species of the genus Marmota are initial hosts of I. crenulatus, they retain the main role as hosts in Eastern-European and Asian morphotypes: C, D, E, F — M. bobac bobac (Muller, 1776), G — M. baibacina centralis (Thomas, 1909), I, J — M. sibirica sibirica (Radde, 1862). The most aberrant in all characters morphotypes are the ones inhabiting European areas (A, B), where marmotes are exterminated. These morphotypes parasitize on hibernating carnivores.

In experiments, the mean life duration of fleas Leptopsylla segnis on white mice (abundance of fleas within natural limits, up to 10 fleas per mouse) was 22.7 days in females and 18.8 day in males. Maximum life duration was 51 and 37 days respectively. In cases, when the initial numbers of fleas were 20 and 28—34 fleas, the duration of life was decreased. The maximum limit decreased greater than the mean duration of life. A survival dynamics of fleas depended upon the flea number. It was found out, that in cases of high abundance of fleas in the beginning of experiments, the mortality rate of males was lower than in females. During the stay on a host the fleas lost gradually an ability to endure a starvation. Possible mechanisms of the regulation of flea abundance are discussed.

Significant karyotipic differences between the tryploid (3n = 9) Prosimulium macropygum Lundstrom and the diploid (2n = 6) P. macropygum ventosum Rubzov have been found. It is suggested to consider the latter taxon as an independent species P. ventosum.

The historical account and recent taxonomic consideration of the monogenean subfamily Encotyllabinae are given. Main criteria used for diagnostics of the subfamily and its genera are discussed.

There are 5 species of the genus Dicranotaenia recorded in the North-west of Chukotka: Dicranotaenia coronula (?) (Dujardin, 1845), D. fallax (Krabbe, 1869), D. parvisaccata (Shepard, 1943), D. sacciperum (Mayhew, 1925) and D. clangulae sp. n. Brief characteristic of the new species. Rostellar hooks 24—28, their total length 17—20 mkm, blade 10—12 mkm, guard processus 7—9 mkm, basis 13—15 mkm. Blade is parallel to basis, significantly longer than guard processus, hook form of aploparaksoid type. Cirrus pouch with s-like bend and distal enlargement (as in D. fallax). Cirrus almost cylindrical, 115—136 mkm in length, diameter at base 20—23 mkm, distal diameter 13—16 mkm. Internal acessory sac (SAI) situated anterior and dorsal from base of cirrus, gradually attenuate to distal end, length of SAI 27—30, diameter at base 30—32. Descriptions of adult forms and metacestodes of four other species are given.

The host variability of adults of Phyllodistomum umblae and P. folium in fishes of the Baikal region has been studied. The absence of considerable geographical variability of P. umblae in Coregonus lavaretus (lake Baikal, lake Storsjon in Sweden) has been shown. The complexes of most stable (the size of acetabulum and eggs, location of acetabulum, ovary and vitelline bodies) and variable (the distance from oral sucker to intestinal bifurcation, distance from intestinal back edge to body end, width of seminal vesicle) features of trematodes of the genus Phyllodistomum have been determined.

The data on a distribution of nematode species of the families Steinernematidae and Heterorabditidae in the northern and southern regions of the Russian Federation are given. Morphological characters of geographic isolates of some nematode species are not different, while their invasive activities in dependence of a temperature are different. Steinernema feltiae occurs in all climate zones, S. carpocapsae is restricted in distribution, nematode species of the family Heterorabditidae were found in southern regions only.

Macrogametes and oocysts of the coccidium Alveocystis intestinalis parasitizing in priapulids have been examined. It is shown, that the macrogametes of this species have small granules of glycoproteinous nature. These granules are wall-forming bodies. It was considered these bodies are absent in A. intestinalis. The oocysts in the investigated material contained mainly 4 sporozoites. The oocysts with 8 sporozoites, which were observed to be most frequent in the materials used for the original description of A. intestinalis, is not a standard for this species. Therefore the joining of A. intestinalis and Pfeifferinella gugleri into one genus based on a similarity of their oocyst structure is incorrect. This conclusion is also supported by the long evolutionary and ecological distances between hosts of these species.

New family name Pillainidae Kazatchenko fam. nov. is suggested instead of family name Channaculinidae. The keyes of the families and genera of fish parasitic copepods of suborder Cyclopoida are given.

The data on species composition, ecology and distribution of snails of the subfamily Planorbinae (tribes Planorbini, Segmentinini) in water basis of Tsentral'noye Polesye are given. In 13 species belonging to the genera Planorbis, Anisius, Choanomphalus, Segmentina, Hippeutis, the parthenites and larvae of Paramphistomum ichikawai Jamaguti has been recorded. Infection rate varies from 0.5 to 18.5%.

The experimental study of life cycles of the trematodes Nenimandijea kashmirensis Kaw, 1950 and Pleurogenoides medians Olsson, 1876 was carried out. It was found out, that their life cycles include: the first intermedial host — the mollusc Boreoelona contortrix ussuriensis, the second intermedial host — dragonfly larvae of the genus Cordulia, and the final host — the frogs Rana nigrimaculata and R. semiplicata. Based on obtained data it is suggested, that Pleurogenoides japonicus (Yamaguti) should not be considered as a separate species.

Summary is absent.

Summary is absent.