AbstractThe study of diatoms (Bacillariophyta) collected in 2000–2001 with a plankton net from the hypersaline Kara-Bogaz-Gol Bay revealed 13 species and varieties of the genera Campylodiscus, Cocconeis, Diploneis, Epithemia, Navicula, Nitzschia, Planothidium, Surirella, and Tryblionella. These diatoms occurred at water salinities from 44 to 250‰. Consequently, salt-tolerance limits were extended for most taxa: 11 species and varieties previously considered in the literature as less salt-tolerant appeared to be halophilous. Planothidium cf. lanceolatum, Epithemiasorex and Nitzschia dissipata differed from their diagnoses by a smaller number of striae on their valves (9–11, 10–12 and 25–35 striae in 10 μm, respectively). P. cf. lanceolatum, Surirella brebissonii var. kuetzingii, and Tryblionella compressa are the first records for the Caspian Sea.