Early reproductive effort and growth were measured in 3 groups of the Pacific oyster Crassostrea gigas: 1 diploid group and 2 triploid groups resulting from either chemical induction (3nCB) or crosses between tetraploid and diploid parents (3nDT). Oysters were reared under intensive nursery conditions and sampled when 5 mo old. Reproductive effort was estimated by cross-sectional area measurements of the visceral mass (i.e. gonadic occupation) and maturation stage was assessed by qualitative histology. As expected, comparison of the reproductive patterns of these 3 groups revealed a lower reproductive effort in triploid individuals relative to diploid. However, gonadic occupation in triploid oysters was higher than expected in both 3nCB and 3nDT groups, as the gonadic occupation was 47% of that in diploids at the sampling date. Our results suggest that, despite much lower mature gamete production in triploid oysters relative to diploid, their reproductive effort can be significant, even in young individuals. Additionally, a significant relationship was observed between gender and body mass within each group and for gonadic occupation in the diploid group, suggesting that there is a link between sex and fitness-related traits in C. gigas.