Comptes Rendus de l' Academie des Sciences Serie III, Sciences de la Vie 325 3 231 238 Paris, France: Editions Scientifiques et Medicales Elsevier SAS.

The French mollusc production is mainly based on the Pacific cupped oyster, Crassostrea gigas. Since 1991, outbreaks of mass mortality of juveniles have been reported during the summer period. These outbreaks are a major concern for the oyster industry. Several studies have established given bacterial strains to be pathogenic for bivalve species, including oysters. Here, we present a study of mortality outbreaks of C. gigas, as initiated in 1995. In a first step, bacterial strains were isolated during the mass mortality outbreak; isolates were then biochemically characterized. Among the isolated strains, some strains of Vibrio splendidus biovar II were found to be pathogenic by means of experimental challenge of oyster juveniles. In the second step, a genotypic identification of the pathogenic strain was undertaken, based on 16S RNA sequences and phylogenetic analysis. It confirmed that the pathogenetic strain belonged to Vibrio splendidus biovar II.