The examined material is deposited in the Limnological Institute, Siberian Branch of the Russian Academy of Sciences (LIN SB RAS).3. ResultsExtensive collection of parasitic annelids (Hirudinea, Clitellata) and their hosts has been going for many years of research in the Chivyrkuy Gulf. Morphological analysis of this collection has been completed recently. The results are presented as a checklist of leech species inhabiting the Chivyrkuy Gulf of Lake Baikal. The list was composed of 22 species and 2 subspecies. The exact systematic position of each taxon and its taxonomic hierarchy data are given in Table 1.Table 1Leech species composition of the Chivyrkuy Gulf with exact systematic position of all taxa and their taxonomic hierarchy data.3.1. Taxonomic Review and a Brief Description of Each TaxonIn a brief commentary, we include a concise description of each leech species with emphasis on host-parasite relationship, zoogeographical and ecological characteristics, the occurrence of species, and its spatial distribution within the Chivyrkuy Gulf. Numerical evaluation of biodiversity could be useful for understanding the importance of species number in terms of actual biodiversity of parasites. The classification of fish taxa is given according to Bogutskaya and Naseka [23].3.1.1. Theromyzon maculosum (Rathke, 1862) Local host: unknown. Locality: Monakhovo Bay (NGR).A widespread but rarely found Palaearctic species. It is known as the bloodsucker of swimming birds inhabiting warm shallow gulfs of Lake Baikal [18, 24]. Two individuals of this species were found in the coastal part of transect 1 (Figure 1) at a depth of 1�C3m during zoobenthic study. The body size of collected specimens was 20�C25mm.3.1.2. Theromyzon tessulatum (M��ller, 1774) Local host: unknown. Locality: Chivyrkuy Gulf (?).This Palaearctic species is known as the bloodsucker of waterfowl [24]. It was not recorded in samples from the Chivyrkuy Gulf. Nevertheless, there are all suitable environmental conditions for existence of T. tessulatum in the Chivyrkuy Gulf.3.1.3. Hemiclepsis marginata (M��ller, 1774) Local host: young fish, invertebrates (NHR). Locality: Kotovo Bay (NGR), Zmejovaya Bay (NGR).A common Palaearctic species. Bloodsucker of fish, tadpoles, and amphipods. Representatives of this species were found at transect 1 (Figure 1) in washout from aquatic vegetation and at transect 2 (Figure 1) on stones at a depth of 0.3�C0.7m. Living leeches are green with a length of 14�C16mm and 3mm in width. Alcohol fixed specimens rapidly lose their beautiful intravital colouring.3.1.4. Helobdella nuda (Moore, 1924) Local host: Mollusca: Gastropoda. Locality: Zmejovaya Bay (NGR).A Palaearctic species inhabiting the Amur River basin. It has been recorded recently in Lake Baikal [25]. We found H.