Massive mortalities of Crassostrea gigas spat associated with OsHV-1 virus occurred every year since 2008, all along the French coasts. Previous experimental studies lead to identify physiological functions of the oyster metabolism that responds to infection, denoting infection routes and symptoms or indicating immune response. Among these functions, studies highlighted several genes whose expression was significantly down or upregulated before the mortality event. The expression of these genes should be then used as predictive indicator of oyster response to pathogen infection, including marker-associated selection. In this study, two experiments were designed to identify genes linked to early infection response of Pacific oyster spat during in situ mortality. For several oyster batches, we looked for statistical associations between, on one hand, in situ variation of candidate gene expression, and, on the other hand, variation in kinetic or intensity of the mortality outbreak.