Rearing of Crassostrea gigas is the most economically important aquaculture activity in France. Bacteria belonging to Vibrio genus constitute one of the most abundant bacterial group in marine ecosystems. Two Vibrio species, V. splendidus and V. aestuarianus, were associated with many cases of mortality events in reared C. gigas spat and juvenile oysters, commonly during summer. This summer mortality syndrome has been extensively documented as the result of complex interactions between pathogens, host and environmental conditions. This work aims to study the Vibrio-host interactions and their modulations according to the virulence of pathogens and the genetic and/or physiological parameters of the host. These factors will be characterized by in vitro and in vivo approaches using virulent bacterial strains, wild or mutated on virulence gene(s) candidates and/or expressing both Green Fluorescent Protein (GFP) and resistance to chloramphenicol in one hand, and genetically distinct oysters families in the other hand. The localization and the quantification of pathogens in different host tissues during an experimental infection survey to study pathogenesis of vibriosis by flow cytometry, immunohistology, electronic microscopy and quantitative PCR. Since hemocytes are the only known immunocompetant cells in oysters, hemocytes-Vibrio interactions will be characterized focussing on phagocytosis reactions. The effect of gametogenesis on oyster susceptibility to vibriosis will be evaluated since some field study data suggest that mature, ready to spawn, oysters are more susceptible to vibriosis. For this goal experimental infections will be conducted on genetically controlled populations of oysters presenting different levels of ploidy and at different time of the year since gonadic maturation is strongly dependant of environmental factors. The hypothesis of an increased transmissibility of vibriosis from infected oyster to healthy one when oysters are highly mature will be also tested by in vivo experimental challenges. Finally the genetic determinism of oysters susceptibility to vibriosis will be explored searching for genetically divergent oysters families presenting contrasted responses to experimental infections with virulent Vibrio. This last approach will allow to consider strategies for genetical improvement of the breedings.