T he cause of Eriocheir sinensis disease, w hich w as called as .. Trembling Disease.. by the f ish farmers, w as studied. Ex aminat ion by electron microscopy show ed that there w ere a numbers of v iral part icles in the t issues of heart、abdominal nerve ganglion、g ill、intestine and hepatopancreas and there was no envelope outside v irion. T he diameter of virion was approx inately from 28nm to 30nm. Virions occurred in the cy toplasm and did not form the inclusion body. According to the characters of viral morphology and dyeability, the virus belonged to Picornaviridae. The healthy Eriocheir sinensis after injected w ith f iltered fluid from ground tissues of diseased Eriocheir sinensis appeared the same symptom of legs t rembling as that of the natural diseased one. This result conf irmed that the picornavirus was pathogen of the disease. The histopathological changes: cells oedema, karyolysis or karyopyknosis, mitochondria oedema, mitochondria cristae broken、curved and necrolysis. The necrotic cells became the substances of no cell st ructure in the severe necrot ic areas.