Quantitative and qualitative bacteriological studies were carried out on vendace larvae and fry, in tank water in the course of fish rearing, and in fish feeds. Quantitative studies comprised bacteria indicatory of water pollution and sanitary state. Qualitative analyses paid attention to bacteria belonging to the different genera and to the family Enterobacteriaceae. The highest number of bacteria was observed for the groups of psychrophilic (TVC 20 degreesC - total viable count of psychrophilic bacteria on broth-agar after 72 h incubation in 20 degreesC), mesophilic (TVC 37 degreesC - total viable count of mesophilic bacteria on broth-agar after 24 h incubation in 37 degreesC), proteolytic and ammonifying microorganisms. Qualitative studies were used to reveal that bacteria from the genera Aeromonas and Flavobacterium, and from the family Enterobacteriaceae were present in fish, while tank water contained bacteria belonging to the genera Pseudomonas, Bacillus and Flavobacterium, and fish feed - those from the genera Aeromonas, Pseudomonas, Bacillus.