A digenic trematode Bucephalopsis haemeanas was observed in the gonads of the backwater oyster Crassostrea madrasensis of the Pulicat Lake. Maximum percentage of infection was 17.7 during December. Highest incidence of infection was observed in the size groups of 60-69 mm and 100-109 mm. Initially the infection was noticed in the gonads of the oyster and later it was found to invade the other tissues such as mantle, gill and digestive gland. Infection was found to be more in the partly spent and spent individuals than in the fully ripe and individuals with gametogenic activity. As a result of infection, gametes are resorbed quickly and thus oysters are castrated leading to indeterminate stage. In the majority of parasitized oysters, feeding was noted to be very poor. The infection seems to be seasonal and starts when the salinity and temperature in lake decline considerably after the heavy rains of North-East Monsoon. The parasites were observed to die at the salinity below 2.3 %_o,