Progeny of Crassostrea virginica oysters, introduced into France in 1992, were reared at 3 different locations. A survey was taken of these young C, virginica oysters during 1993. Histological and electron microscopical examination revealed a parasite interpreted as a Marteilia-like protozoan in 1 oyster. The structure and the initial stages of the developmental cycle of this parasite are described for the first time in American oysters. However, no mature spores were detected. Thus, exact identification of the protozoan is difficult. This study indicates that American oysters could be infected with Marteilia spp, during cohabitation with Marteilia-infected native bivalves in France.