A number of bivalve species worldwide, including the Pacific oyster, Crassostrea gigas, have been affected by mass mortality events associated with herpesviruses, resulting in significant losses. A particular herpesvirus was purified from naturally infected larval Pacific oysters, and its genome was completely sequenced. This virus has been classified as Ostreid herpesvirus 1 (OsHV-1) within the family Malacoherpesviridae. Since 2008, mass mortality outbreaks among C. gigas in Europe have been related to the detection of a variant of OsHV-1 called Var. Additional data are necessary to better describe mortality events in relation to environmental-parameter fluctuations and OsHV-1 detection. For this purpose, a single batch of Pacific oyster spat was deployed in 4 different locations in the Marennes-Oleron area (France): an oyster pond ("claire"), a shellfish nursery, and two locations in the field. Mortality rates were recorded based on regular observation, and samples were collected to search for and quantify OsHV-1 DNA by real-time PCR. Although similar massive mortality rates were reported at the 4 sites, mortality was detected earlier in the pond and in the nursery than at both field sites. This difference may be related to earlier increases in water temperature. Mass mortality was observed among oysters a few days after increases in the number of PCR-positive oysters and viral-DNA amounts were recorded. An initial increment in the number of PCR-positive oysters was reported at both field sites during the survey in the absence of significant mortality. During this period, the water temperature was below 16°C. Since the first report by Farley et al. (1), herpesviruses have been associated worldwide with mortality events resulting in significant losses in a number of bivalve species, including the Pacific cupped oyster, Crassostrea gigas, in both hatcheries/nurseries and the natural environment (2-11). A herpesvirus was purified from naturally infected larval Pacific cupped oysters collected in 1995 in a French commercial hatchery (12). Its genome was completely sequenced (13), and the virus was classified as Ostreid herpesvirus 1 (OsHV-1) within the family Malacoherpesviridae (14, 15). Moreover, since 2008, mass mortality outbreaks among Pacific cupped oysters have been reported in Europe, including France, Ireland, the Channel Islands, and the United Kingdom (16-23), in relation to the detection of a particular genotype of OsHV-1 called Var (24).A protocol based on the intramuscular injection of 0.22-mfiltered tissue homogenates prepared from naturally infected spat was developed (25). The results of experimental trials showed that mortality was induced after the injection. Furthermore, analysis of injected oyster spat revealed large amounts of OsHV-1 DNA by real-time quantitative PCR. In this context, OsHV-1 was inferred to be the causative agent of the mortality reported in the study (25). In addition to transmitting OsHV-1 infection by intramuscular inoculation, waterborne transmission to healthy oyster sp...