The "Appendage Shaking Disease" (ASD) of Eriocheeir sinensis has been one of the most serious disease in the farmed freshwater crabs of China in the past five years.The ultrastructural study on the affected crabs sampled from Huaiyin district, Jiangsu Province indicated th at massive Richettsia-like organisms (RLO) had been found in all th e affected crabs.However, no RLO was found in healthy crabs sampled from East Taihu Lake, Jiangsu Province where there was no epidemic of ASD.The RLO that have mainly infected the periopod muscle organization and blood cell of crabs and amage th periopod muscle fiber of muscle cell and, hence, be harmful to the locomotion system of crabs.This was identical with the symptom of ASD.So we come to the conclusion that RLO is probably the pathogen of ASD.