A virulent strain of motile aeromonad (77-18) differed from an avirulent strain (78-16) in the contents of lipids and phospholipids and odd-numbered fatty acids, activity of hydrolytic enzymes, and amount of proteins with molecular weights of 47–56 kDa. It is assumed that proteins with molecular weights of 47–56 kDa, proteolytic enzymes active within a wide pH range, and odd fatty acids may act as pathogenicity factors. Each of these compounds or their combination determines a certain stage of infection.