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In the basin of the Kama River, in upper Vychegda River, and in tributaries of the upper Northern Dvina River, the parasite fauna of the minnow is monotonous and component communities of parasites are mature (balanced, according to Pugachev, 1999). They are completely formed in June.

Copepoda parasitica of Baikal fishes (16 species) is divided into 7 groups according to their localization: parasites of the gill apparatus, gill covers, gill and buccal cavities, nasal fossa, cutaneous covering, and fins. It was proposed to separate nasal fossa parasites as the special ecological group of mesoparasites. Typical speciemens of the group include crustaceans Salmincola longimanus complex — parasites of grayling and cisco fishes consist of three species (S. longimanus, S. svetlanovi, S. lavaretus) and one subspecies (S. longimanus sibirica).

New data on Ospey parasites in Karelian Republic are given. One specimen was investigated. Two parasite species — Nematostrigea serpens and Diplostomum pseudospathaceum were found. Trematoda D. pseudospathaceum was recorded in Osprey parasite fauna for the first time.

Data on trematodes of Plagiorchiida, Renicolida, Strigeida, and Schistosomatida, parasitizing in birds of the Middle Volga region are given. Proceedings of the different authors are supplemented by results of our research. Two species of trematodes (Brachylaima mesostoma and Mosesia amplavaginata) are specified for birds of the middle Volga region for the first time. New hosts were revealed for 2 trematode species: the great tit for Plagiorchis laricola and the common chaffinch for M. amplavaginata. The following characteristics for each parasite are given: it's systematic position, the spectrum of hosts, localization, collecting site, biology, the degree of host specificity and geographical range. The morphological description and original figures of 11 species of trematodes are presented.

The histological study of the trematoda sporocysts Leucochloridium paradoxum confirmed the presence of three morphological zones in it: 1) central part (reproductive), where embryos are forming, 2) narrow tubes through which the embryos penetrate in colored broodsacs (3), where the development of metecercaria completes. It was found that germinal mass only is the reproduction organ of the sporocysts, located in reproductive zone. There are young (without embryos), mature (with embryos) and degenerated germinal masses. So, in the process of sporocysts development the centre of multiplication of germinal elements was changed. The old parts of central part are degenerated, but the new ones with young germinal masses appear. The multiplication of generative elements does not take place in the broodsacs which are breeding cameras functionally.