The methionine-enkephalin(M-ENK)in the body surface and digestive system of Neverita didyma was analyzed by the immunohistochemical S-ABC(strept avidin-biotin complex) staining.The results showed that M-ENK positive cells existed widely in the epithelium of foot,inner and outer epithelia of mantle,mucous membrane epithelium of oesophagus,stomach and intestine.Besides,the positive immuno-reaction was centralized in the dissociative sides of alimentary canal epithelium.A small quantity of positive cells was observed in the esophageal gland,too.A great number of positive cells were observed in the liver,mostly in the dissociative sides of the gland cells.It can be concluded that M-ENK is abundantly distributed in the epidermis of the body surface and digestive system in N.didyma,and that different densities of M-ENK in these organs may be referable to their different functions.