Pathogenic infect ion status of the cultured scallop Chlamys farreri was invest igated from May to November 2001. With negative staining technique, virus and Rickettisia-like organisms ( RLO) were detected in tissue extract of samples. Infection rate of virus ranged from 80% to 100%. The highest infection intensity of virus is found in July and August when acute death of Chlamys farreri occurred. Similar results acquired after Chlamys f arreri was injected with virus. The present work demonstrated that infection rates and intensity of RLO were not signif icantly different for cultured and artificial infected Chlamys farreri . The observation in 98 specimens conf irmed that the direct pathogen of explosive epidemic disease of Chlamys farreri was possibly a kind of spherical RNA virus with envelope and spike in cytoplasm. Which was found in a large amount during who le culture period and throughout all part s of the host .