This study was conducted to investigate Serratia liquefaciens infection in juvenile cultured rainbow trout (Oncorhynchus mykiss Walbaum) during June and July in 1996 about 20 days after flooding in three farms in Erzurum. Bacterial isolates were identified from each farm and tested to determine sensitivity against 18 chemotherapeutants. In addition to minimum bactericidal concentrations (MBC) of chloramine-T and potassium permanganate (KMnO4) were exposures of 17.07-20 mg/1 for 1 h and 23.27 mg/l for 10 min. Infections caused fallen scales. bloody and swollen kidney, hyperaemic and pale regions in the liver, haemorrhagic spots in the gills and bloody exudate in the intestine. Histopathological examination demonstrated pathological changes in the liver, spleen and kidney. Naturally infected fish were examined for chemical parameters of blood and compared with healthy fish. Amylase and glutamate oxalacetate transaminase (GOT) enzymes, triglyceride (TG), cholesterol (CHOL), albumin (ALB), albumin/globulin (A/G). iron (Fe) levels, total iron binding capacity (TIBC), unsaturated iron binding capacity (UIBC) and saturation (ST) in the blood serum of infected fish were significantly increased. Glutamate pyruvate transaminase (GPT) and L-lactate dehydrogerase (LDH) enzymes, bilirubin (BIL), glucose (GLC), creatinine (CRE) and total protein (TP) values were not significantly different between the naturally infected fish and the healthy fish. In the therapeutic applications, oral applications of potentiated sulphonamide after disinfections with chloramine-T controlled the infections.