INTRODUCTIONThe Altai osmans of g. Oreoleuciscus (Cyprinidae)are the most numerous inhabitants of the Central-Asianinternal basin of the Western Mongolia. Earlier, basedupon the analysis of living traits and morphology, sev-eral morpho-ecological forms were distinguishedamong the Mongolian Altai osmans, some of whichwere previously ranked as species (Dgebuadze, 1982;Baasanzhav et al., 1983; Borisovets et al., 1985). Thus,in the lower reaches of the Kobdo River (Ogon-NurLake), three forms of the Potanin’s Altai osman (Oreo-leuciscus potanini) were described. Conventionally,these forms were named as the “phytophagous”,“sharp-nosed,” and “piscivorous” (Baasanzhav et al.,1983). Despite presence of numerous intermediateforms, the adult specimens of the above forms are eas-ily differentiated with consideration of morphologicalfeatures, patterns of distribution in a water body, andfeeding spectra (Baasanzhav et al., 1983; 1985). Thefeeding base of all three forms is composed of thecharophytes. However, in the phytophagous form, ashare of insect larvae and imagos is comparatively high,while other groups of aquatic organisms, including fish,are represented to a lesser extent. In the food of thesharp-nosed form, the planktonic algae and inverte-brates made up a considerable share, while in the pis-civorous form—fish (the osmans, and sometimes theloaches Ortrias sp.). The young osmans feed predomi-nantly on the planktonic organisms while in the stom-ach content of the phytophagous form, the share of thebenthic organisms is quite considerable, making upfrom 16 to 53% of the mass (Baasanzhav et al., 1985).It seems obvious that, during the process of morpho-ecological diversification of the Altai osmans, the selec-tion of particular food items played an important role.Consequently, the question arises if there are any differ-ences in the activities of digestive enzymes that digestfood proteins and carbohydrates in different forms ofosmans and at different stages of their ontogenesis. The presence of specific feeding preferences in thisvery variable group of fish (while the feeding spectraare really wide, from charophytes to fish) suggests thepresence of adaptations in the digestive enzymes' sys-tem to the biochemical composition of food. However,we do not know of any published data concerning theactivities of the digestive enzymes processing the mainplastic and energetic food components in the Altaiosman intestines.The goal of the present paper is to determine theactivities of digestive hydrolases (carbohydrases andproteinases) in the intestinal mucus of the three formsof Altai osmans from the Mongolian Nogon-Nur Lake.MATERIAL AND METHODSThe material was collected during the field studiesof the ichthyological detachment of the Joint Russian-Mongolian combined biological expedition of the Rus-sian Academy of Sciences and the Academy of Sci-ences of Mongolia at the end of August 2004. The fishwere collected in the Lake Nogon-Nur that is a part ofthe lake system Hara-Us-Nur–Hara-Nur–Durgun-Nur,at the River Kobdo downstream (Western Mongolia).Young and mature specimens of the phytophagous(56 ± 3.0 and 116.4 ± 7.0 g in weight), sharp-nosed(17.3 ± 1.3 and 109 ± 11 g) and piscivorous (116 ± 14and 589 ± 30 g) forms of the Potanin’s Altai osman.The fish were caught by nets (10 mm to 55 mm meshsize) from which they were removed every 8 h. Theintestines were removed immediately after the catch,