Mollusc aquaculture is an important commodity for France with Pacific oysters, flat oysters and the great scallop being the three main species. Pacific oyster larvae production is currently successful, however larval production of flat oyster and scallops regularly face problems remedied by antibiotics. Using a bioassay technique, four potential probiotic strains (Alteromonas macleodii 0444, Neptunomonas sp. 0536, Phaeobacter gallaeciensis, Pseudoaltermonas sp. D41) were tested upon the larvae of the three mollusc species when challenged with pathogens. Pacific oysters and flat oysters were challenged individually with the pathogens Vibrio coralliilyticus and V. pectenicida, while scallops were challenged with V. coralliilyticus and V. splendidus. Survival of the larvae was measured in challenged larvae given the probiotics and those without probiotic. In scallop larvae, protection against V. coralliilyticus and V. splendidus was provided by prior administration of P. gallaeciensis (29% and 53% better than the respective pathogen controls), A. macleodii 0444 (30%, 36%) and Neptunomonas sp. 0536 (36%, 55%). Significant protection with Pseudoalteromonas sp. D41 was only provided against V. splendidus (35%). In flat oyster larvae, protection against V. coralliilyticus and V. pectenicida was provided by P. gallaeciensis. (73%, 69%) and A. macleodii 0444 (45%, 53%). Neptunomonas sp. 0536 provided significant protection only against V. pectenicida (48% higher). Use of Pseudoalteromonas sp. D41 did not offer significant protection for flat oyster larvae against either pathogen. Experiments with Pacific oysters suggested that Pseudoalteromonas sp. D41 and P. gallaeciensis could be effective against challenge by V. coralliilyticus (50% and 40% improved survival respectively) but not against V. pectenicida. The results indicate that P. gallaeciensis, A. macleodii 0444, Neptunomonas sp. 0536 and Pseudoalteromonas sp. D41 could be useful in mollusc larviculture.