Abstract: The enteritis of two-year-old grass carp is caused by the motile Aeromonas. This paper describes a series of tests and observations in relation to the pathogenesis of enteritis, including the comparison of densities and toxicities of Aeromonas in intestine and blood between di seased and healthy fish, the effect of water temperature on the pathogenicity of the bacterium, the relative abundance of Aeromonas in different organs of the diseased fish and the artificial infection of this disease by inoculating the bacterium and its cell extract respectively. Tachypleus Amebocyte Lysate (TAL) has been used to determine the endotoxin in blood of the diseased and the healthy fish. The pathogeny of bacterial enteritis may be summarized as follows:When water temperature rises over 18℃, the pathogenic Aeromonas in the digestive tract begins to proliferate rapidly and causes functional disorder of the capillaries of the intestinal wall. After entering the blood stream, this bacterium undergoes further proliferation and spreads all over the visceral tissues. Owing to the self-dissolution of the bacteria, bacterial endotoxins are released, which alter the permeability of the blood vessels and eventually result in septicaemia of the grass carp.