In some instances, from appearently healthy fish internal organs various bacteria were isolated. Schäperclaus and Mann (22) found that at suboptimal temperatures (0-10°C) carp had asymptomatic infections with bacteria belonging to the genera Aeromonas and Pseudomonas. Bisset (5) was the first to investigate the effect of temperature on the presence of saprophytic water bacteria in the fish tissues. He found that at 10°C, or below, fish frequently contained bacteria in their tissues without becaming diseased. According to Evelyn and Mc Dermott (10) baderia representing genera Pseudomonas, Aeromonas, Achromobacter, Alcaligenes, Flavobacterium, Aerobacter, Escherichia, Paracolobacterium, Lactobacillus, Micrococcus, Bacillus and Brevibacterium were isolated from various fresh water fish internal organs. Similar bacteria were also isolated from white perch (Roccus americanus) in an estuarine environment (Allen and Pelzar, 1). Bullock and Snieszko (7) isolated from blood and kidney of healthy trout low numbers of bacteria representing the genera Pseudomonas, Aeromonas and Flavobacterium. In adition, sera from these trout had agglutinins against some of the isolated bacteria, and bacterial types found in trout were the same as those cultured from race way water in which fish were raised. To further extent these observations to Poland, bacteriological examinations of the kidney, spleen and liver of appearently healthy carp (Cyprinus carpio) were undertaken with a purpose to determine the kind and number of live bacteria present in these organs. The results were to enlighten the fish bacterial disease ecology.