The histopathology of the Meretrix meretrix Linnaeus suffering from red muscles disease was studied by histological and histochemical staining methods. The main characters of diseased clam are that the histological structures were irregularly changed, the epithelial cells swelled and detached. Some abnormal structures were observed in tissues of gills, mantles, digestive diverticulum, etc. , e. g. strong basophilic granules, acidophilic granules and protozoan. Histochemical studies, on the other hand, indicate that amount of amylose and activity of phosphatase appear obvious changes, e. g. the content of amylose increases in the absorptive cells of digestive diverticulum and intestine. The activity of alkaline phosphatase ( AKP) increases and the acid phosphatase ( ACP) decreases in the tissue of digestive diverticulum and digestive tract, while in the gill tissues, the activity of ACP increases and AKP decreases.