Diatoms of periphyton in the mountain lakes were studied. In total 119 species and varieties from 42 genera, 21 families were identified in four Mezhgornye lakes and Nydysey river. The families Fragilariaceae (22), Euno- tiaceae (14), Achnanthidiaceae (13), Gomphonemataceae (11 taxa) and genera Eunotia (14), Gomphonema (10 taxa) are the most diverse. Ecological analysis shows the predominance of halophobic and circumneutral diatoms in diversity. Arctic-alpine species (34 %), indicators of distrophic-oligotrophic conditions (36 %) and clean water (47 %) form the considerable part of diatom composition. Dominant species in the lakes are Epithemia adnata, Gomphonema acutiuscu lum, Rossithidium linearis, Staurosira venter, Staurosirella pinnata, Tabellaria flocculosa, and in the river - Diatoma mesodon.