A programme of zoosanitary survey of the populations of black-lip pearl oyster (P. margaritifera) has been developed in French Polynesia during 1996 and 1997. Based on histological and ultrastructural examinations, the study of pearl oysters sampled in several pearl farms from the Society Islands, Tuamotu Atolls and Gambier Archipelago, has been particularly turned to enzootic pathological problems. Particular attention has been given to the shell disease and to the necrosis of the adductor muscle, symptoms that had accompanied the mass mortality in 1985.Virus–like particles, about 40 nm in diameter, were detected in membrane bound vesicles from the necrosized muscle tissue. A new Rickettsia-like organism found in epithelial cells of the digestive tubules was described, while parasites previously reported, such as cestodes and gregarines, were recognized in some animals. Apart from the pathology, a thorough study of ‘parasite-like bodies’ discovered in the pearl sack epithelium showed dysfunction in the periostracal secretion.