In late winter 2000, 160 mature individuals of the hermaphroditic giant clam Tridacna maxima (Bivalvia: Tridacnidae) were collected as a random sample from a wild bed inhabiting the coasts of Sharm El-Sheikh, Northern Red Sea, Egypt. Examination revealed that the gonads of 36 individuals (22.5%) were castrated by the sporocysts and cercariaeof a bucephalid trematode species (Digenea: Bucephalidae). Although the prevalence was moderate, its effect on the reproductive capacity of these clams was significant, since the gonads of 21 individuals (13.13%) were heavily infected and completely castrated, while those of 15 individuals (9.37%) were slightly infected and partially castrated. Sporocysts and cercariae were described and figured, and the mechanism of parasitic castration was discussed. It was concluded that such parasitic castration may harmly affect the overall fecundity of the whole population of T, maxima inhabiting the coasts of Sharm El-Sheikh; and possibly other areas the Northern Red Sea.