Complex microbiological and chemical analyses of water and bacterial mats were performed in the Kotelnikovsky Hot Springs (Lake Baikal). Transmission electron microscopy demonstrated that short rods about 1.2-2 microm in diameter predominated in the natural microbial community. Scanning electron microscopy coupled with chemical analysis revealed a characteristic P peak in the bacteria-like mineral particles, which suggests their biogenic origin. Most strains of the thermophilic microorganisms were gram-positive spore-forming rods and can be assigned to the genus Bacillus. Assays for potential enzyme activity demonstrated that most of the strains tested were active at high temperature. The data obtained suggest high activity of the bacterial community in situ and its particular role in the functioning of the hydrothermal ecosystem.