The observed concentrations of bacteria at two large fish farms were not high, but due to the great volume of the discharge the total amount of bacteria was large. Total coliform (TC) bacteria identified belonged mainly to the genera Enterobacter, Citrobacter and Aeromonas. The majority of faecal coliform (FC) strains were Escherichia coli. E. coli was absent, or occurred at very low concentrations, in the influent water, but was present in the effluent water, in the sediment, and at one fish farm also in fish faeces. FC bacteria were not observed in the fish feed. The concentrations of faecal streptococci (FS) in the influent water were low, but strains isolated were identified as group D streptococci. The concentrations of FS were low in the feed and sediment samples but were elevated in the fish faeces and also in the effluent.