The Pacific oyster, Crassostrea gigas, is commonly described a protandrous alternative hermaphroditic species in which simultaneous hermaphrodites are rarely observed. However, its sex determinism is still debated and remains unclear. Genetic sex determinism has been investigated in bi-parental progenies, i.e. offspring obtained by crossing one male and one female oyster. This makes difficult to interpret the results, as it is not possible to distinguish whether the parents are genetically male and female, or sequential hermaphrodites. This study reports the sex ratio at 14 and 15-month-old in the progeny of a simultaneous hermaphrodite oyster. SNP genotyping confirmed that they were produced by selfing. In addition, sex ratio was also recorded in bi-parental control progenies, produced by mating two parents (i.e. one female and one male). When 14-month old, oysters produced by selfing were mainly males (98.8%) while sex ratio of control oysters was close to 1:1. Most interestingly, when 15-month old, 70% of the oysters produced by selfing had or were undergoing a sex change between May and June, and 68.8% of the oysters were simultaneous hermaphrodites suggesting inheritance of this reproductive trait, but also its temporal plasticity. In contrast, control progenies showed less than 1% of simultaneous hermaphrodites, as usually reported in C. gigas. To our knowledge, this is the first report of such high proportion of simultaneous hermaphrodites in C. gigas and of the inheritance of this trait. Our results open new perspectives in the production of inbred lines of Pacific oysters by self-fertilization through successive generations of selfing.