This paper reports the histopathology of Meretrix meretrix Linnaeus parasitized by the larvae of Bucephalidae. The specimens were collected from mollusca-farming ponds in Laizhou, Shandong province in March, 2001. The main symptoms of infected clams is that the visceral mass becomes thin, the adductor muscle is weak and the surface of the shell lacks luster. Histological examination revealed that most of the larvae occupied the genital gland of the clam, with relatively few in other organs. It damages not only the tissues parasitized by the parasites, but also caused the debilitation of other organs. In heavy infestations, the gonads is completely destroyed and replaced by mass of larval digeneans. Other organs exhibited obvious pathological changes, e. g. the epithelial cells detached from the wall of digestive tract, digestive diverticula and gills. The structure of muscular tissues becomes irregularly arranged, with some tissue melted. In the end, all of the essential organs were broken down, which causes the host dead.