An increasing number of hypotheses are being proposed to explain the faster growth potential of triploids in molluscs, including their partial sterility or their higher heterozygosity compared to diploids. Triploid advantage however, remains controversial for poorer sites, because of a potential trade-off with survival. These questions were addressed in Crassostrea gigas by deploying meiosis II triploids and their diploid siblings from a single mass spawning of three males and seven females, in two contrasting locations for their trophic resources. One hundred and fifty individuals were sampled at each site after nine months, measured for weight and biochemical composition, and genotyped using three microsatellite and seven allozyme loci. Higher performance was observed at the fast-growing site for all traits except shell weight, and triploids had greater weights and biochemical contents than diploids at harvest. Triploids also grew faster at the poorer site, and showed similar survival rates to diploids at both sites. Triploids had significantly higher average allozyme and microsatellite diversity. However, they performed better for a wide range of individual heterozygosity values, arguing for an advantage of the triploid state per se, that could be due to positive effects on growth of both sterility of triploids with subsequent resource re-allocation and possible faster transcription with three copies of each gene. Despite evidence of very low or no inbreeding in the diploid sample, positive associations between individual allozyme diversity and growth were detected, which explained little but significant amounts of phenotypic variation. These associations were interpreted as direct effects of allozymes, either alone or including epistatic interactions with other loci. In addition, measures of individual distance (mean-d2) specific to microsatellites, were negatively correlated with growth in diploids, indicating possible effects of outbreeding depression between more distant genomes of parents from distinct populations.