Animals in the aquatic environment carry bacterial flora, which is a reflection of the flora in the environment. Intestinal bacteria, such as Aeromonas sp. and Vibrio sp. often cause opportunistic infections. Conventional approaches, such as the use of disinfectants and antimicrobial drugs, have had limited success in the prevention or cure of aquatic disease. Antibiotics are administered to aquariums primarily to prevent or treat bacterial diseases. The indiscriminate use of antimicrobials calls for a better understanding of the impact of these compounds on the microflora of the environment and the cultured fish. Virulence factors of A. hydrophila, which contribute to their pathogenicity, include the production of endotoxins, extra cellular enterotoxins, hemolysin, cytotoxins and protease, the ability to adhere the cells, and the possession of certain surface proteins. These virulent determinants, most of whose the mechanisms of action remain to be determined, are involved sequentially in enabling the bacteria to colonize, gain entry, establish, replicate and cause damage in host tissues and to evade the host defense system and spread, eventually killing the host. Current studies to analyze the gut micro flora of the ornamental aquarium fish Carassius auratus from infected freshwater ornamental fishes. The present study characterizes the virulence factors of the selected A. hydrophila strains.