Scallop Pecten maximus larvae have been cultured at the Argenton and Tinduff (Brittany, France) hatcheries with antibiotic treatment (chloramphenicol at 8 ppm) for 15 yr. Without treatment, outbreak of disease has normally occurred between Day 12 and Day 19 or sometimes earlier. A bacteriological study of larvae reared with and without antibiotic was performed over a 4 yr period. Among the collected strains, 2 clusters (C and F) of vibrios were present at high densities only in larvae cultured without treatment. One cluster (C) was routinely isolated over the 4 yr of study, while the other (F) was collected only in the third year. Their virulence with respect to scallop larvae and their lack of infectivity with respect to oyster larvae were demonstrated in an exposure experiment. The vibrio F strain tended to lose its virulence after 5 subcultures, whereas the vibrio C strain retained the ability to kill scallop larvae in experimental infections. Three other vibrios isolated in moribund oyster larvae caused mortality in oyster larvae but not in scallop larvae. Different methods were used to determine the taxonomic position of these virulent bacteria. The phenotypic traits of bacterial isolates were determined with the Biolog GN microplate, the API 20E system and the reference method. Patterns of cytoplasmic proteins were identified by electrophoresis in SDS-PAGE. These different methods consistently confirmed the existence of 2 vibrio species pathogenic to scallop larvae. Affiliation of cluster F with Vibrio splendidus was assessed by Biolog tests and by analysis of 16S rRNA sequences. One pathogenic bacteria of oyster larvae was also very close to this second cluster, whereas the 2 others from moribund oyster larvae and cluster C may constitute 2 different species.