Abstract: In the present study Aeromonas hydrophila outbreaks occurred for 3 months (July, September and October) in 1996 about 15-20 days after flooding in juvenile cultured rainbow trout (Oncorhynchus mykiss Walbaum) were investigated. Sensitivities of four isolates of A. hydrophila against 16 chemotherapeutants were determined. Minimum bactericidal concentrations (MBCs) of formalin to the isolates were between 6.4-8 g/ml. 23 g/ml dose of potassium per manganate (KmnO4) and 15 g/ml dose of Chloramine-T were effective to inhibit one of the isolates but not to the other isolates. Oral applications of ofloxacin or flumequine following bath disinfections with formalin controlled the natural infections. Pathogenicity of three isolates of A. hydrophila was tested by intramuscular injection into healthy 1-year-old rainbow trout. The lethal dosages of 50 % end point (LD50) for the isolates were calculated as 4 x104, 2 x105 and 5 x105, respectively. Natural and experimental infections caused dross clinical abnormalities such as abnormal movements, anorexia, darkening skin, pale and swollen gills, cherry-red coloured spleen, necrosis in liver, haemorrhage in kidney and serous exudates in intestine. Histopathological examination demonstrated pathological changes in gill, brain, heart, kidney, liver and intestinal tissues of naturally infected juvenile rainbow trout. Glutamate oxalacetate transaminase (GOT) enzyme and bilirubin (BIL) levels in blood serum of naturally infected rainbow trout were significantly higher than in healthy fish. No significant increases were observed in the serum L-lactate dehydrogenase (LDH), blood urea nitrogen (BUN) and creatinine (CRE) levels of diseased fish group. There was no significant a decrease in the mean concentration of glucose (GLC) of the naturally infected fish group.