We investigated the role of oyster gonadal TGF beta (og-TGF beta) in the reproduction of Crassostrea gigas, using an in vivo RNA interference approach. We designed double-stranded RNA targeting og-TGF beta, which is specifically expressed in the somatic cells surrounding germ cells in the gonad of both male and female oysters. In vivo injection of this og-TGF beta dsRNA into the gonad led to knock-down phenotypes for both sexes, with significant reduction (77.52% relative to controls) of the gonad area, lowered reproductive effort and germ cell under-proliferation. Interestingly, half of the injected females halted their vitellogenesis, since we were only able to observe pre-vitellogenic oocytes. In addition, apoptotic germ cells and haemocytes infiltrated into the gonad, likely as part of the active resorption of degenerating germ cells. Conversely, males showed a normal phenotype at the cellular level, with spermatids and spermatozoids observed in the gonads of control and injected males. As a result, og-TGF beta appears to play an essential role in C. gigas germ cell development by functioning as an activator of germ cell proliferation in both male and female oysters and vitellogenesis in females.