The proliferation of the voluntarily-introduced cupped oyster, Crassostrea gigas, has attained the proportions of species invasion in many intertidal habitats in Europe, presumably resulting from successful reproduction of farmed individuals. It is thus imperative to better understand the reproductive characteristics of farmed oysters, since they are directly under human control. We quantified the dry tissue mass (DTM), gametosomatic index (GSI), and reproductive cycle of farmed oysters at two sites in Bourgneuf Bay, France, in relation to environmental parameters using continuously-recording probes in 2005 and 2006. The GSI was developed for this study, based on the actual area occupied by gametes, rather than the area of the gonad previously used for quantitative histological estimation of reproductive effort. The two sites, intermediate - (IT) and high-turbidity (HT), differed markedly in the amount and quality of particulate suspended matter, and also in fine-scale temperature variations. Oysters at both sites presented two spawning periods in both 2005 and 2006; Bourgneuf Bay is thus near the northernmost European limit for a 2-spawning cycle in Crassostrea gigas. Gonad maturation was initiated when spring water temperature reached 8-10 degrees C, and gamete atresia occurred when water temperatures transiently dipped to 15-18 degrees C. Spawns, which occurred above 18 degrees C, were timed by fine-scale water temperature variations. Particulate organic matter quality peaks, coinciding with gonad maturation, were related to DTM variations before spawning periods, for the IT oysters in both years, and for the HT oysters in 2006. The reproductive effort (GSI) of oysters was similar at both sites; however, the fates of the gametes differed according to site. At the first spawning, the IT oyster gamete emissions were +1 month delayed, as were peak water temperatures greater than 18 degrees C, and more pronounced, compared to the HT site. Although the second spawning showed high proportions of atretic oocytes at both sites in both years, the IT oysters evacuated twice as many gametes as the HT oysters in 2005. The IT conditions therefore appear more suited to Crassostrea gigas gamete evacuation than the HT conditions.