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This study was carried out for the investigation of the mortalities observed in Black Sea trout (Salmo trutta labrax) reared in a fish farm in Marmara Region, detection of pathological changes in tissues caused by agents and also for determination of proper treatment against isolated pathogens. For this purpose, six moribund Black Sea trout (400-500 g) were examined by using bacteriological, parasitological and histopathological methods. Externally, affected fish showed darkening of skin, deep skin lesions with exposure of muscle around the caudal fin as well as adipose and caudal fin erosions also the lesions down into the skeletal muscles around the caudal fin; internally pale liver, hyperemia around the gastrointestinal tract. Both external and internal parasites were not found on the affected fish samples. Bacterial inoculations were made from the visceral organs such as liver, spleen and kidney. Depending on the morphologic, physiologic and biochemical properties, bacterial isolates recovered from the fish samples were identified as Pseudomonas fluorescens and Staphylococcus sp. It was also determined that these isolates are the most sensitive to flumequine. Histopathologically, hemorrhages between the pyloric ceca; sloughing of the epitelial cells of mucosa to the lumen; degeneration of the parenchyma epithelial cells of the kidney tubules, periglomerular oedema and depletion of the haemopoietic tissue; degeneration of the liver parenchyma cells; depletion of red pulp and melanomacrophages center in the spleen and also necrosis in the gill filaments were observed. In this study, Pseudomonas fluorescens and Staphylococcus sp. were isolated and identified from cultured Black Sea trout for the first time as a disease agents as well as detailed pathological changes caused by these pathogenic bacteria in tissues. [ABSTRACT FROM AUTHOR]