The present study forms the first report of Nematopsis sp. infection in the edible oyster, Crassostrea madrasensis from India. The study was carried out as part of a detailed pathological investigation of C. madrasensis along the southwest coast of India. Sporozoites of Nematopsis sp. were found in samples collected from two locations. Light microscopic observation revealed ellipsoidal oocysts measuring 16.63±2.40 µm in length and 11.11± 2.49 µm in width (n=30) in the connective tissues of

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... , mantle, visceral mass and gonads. Prevalence of infection ranged from 11 to 27%. Apparent pathological changes included compression of adjacent digestive diverticulae in visceral connective tissue infections and presence of phagocytosed oocysts in water channels in the case of gill infections. With relevance to the expanding culture of C. madrasensis, monitoring potential pathogens of this species in its natural habitat is important for developing suitable health management packages.