Infectious diseases are the main reason for economic harm in the aquaculture industry, which is depressingly impacted by numerous pathogenic organisms.  Arenomas hydrophila (A. hydrophila) recognized as an opportunistic pathogen.  Forty fish were used in the present study, which was divided into two groups. One serves as control group (n=20) and the other group was experimentally infected with a strain of A. hydrophila. The clinical response was observed every day for 7 days. Then at the end of the experimental period, the fish dissected and the gills were removed for histological investigation. External signs of infection revealed many lesions such as hemorrhage in fins as well as a caudal peduncle. Also, reddish/gray ulcers with necrosis extending to the muscle were observed. Histopathologically, gills of Cyprinus carpio exhibited a varying degree of histopathological changes. Epithelial changes such as lifting and rupture of the lamellar epithelium, lamellar fusion.