This study focused on isolating and diagnosing the pathogenic bacteria in the fish and examining their histological effects on the internal organs, including the liver, kidneys, and intestines of the common carp (Cyprinus carpio L.). In the intestine, necrosis was observed, with dissection of the mucous layer and infiltration of neutrophil cells in the submucosal layer. The liver exhibited hepatocellular vacuolar degeneration with bile duct dilatation and degeneration of its lining. In kidneys, a renal tubule necrosis was observed with epithelial shedding and infiltration of macrophages and neutrophils in the tissue between the tubules. Pseudomonas aeruginosa has been identified as a causative agent through histological methods, and its impacts on the fish health have been documented.