Progeny of Crassostrea virginica oysters, introduced into France in 1992, were reared during 1992 and 1993, to test their growth performances. These assays were performed to investigate the possible replacement of Crassostrea gigas with a non indigenous species, Crassostrea virginica. During the summer of 1993, sporadic mass mortalities (80-90%) occurred among young, Crassastrea virginica, oysters reared in the IFREMER nursery in La Tremblade (Charente Maritime).