Chemical treatments with cytochalasin B were used to induce triploidy in the progeny of a mass fertilization of 3 male and 7 female Crassostrea gigas parents. Triploids were produced either by retention of the first (meiosis I (MI) triploids) or the second (meiosis II (MII) triploids) polar bodies. These animals, together with their diploid siblings, were divided for two experiments. One set was used to compare physiological performance, and the other set deployed to compare growth in two different natural environments. For both experiments, genetic variability in different ploidy classes was estimated using three microsatellite loci and eight allozyme loci. The microsatellite loci were highly polymorphic, allowing independent confirmation of ploidy status and the unambiguous identification of parentage for each oyster. Significant differences in parentage were found between ploidy classes, despite the fact they originated from the same mass fertilization. This indicates that the assumptions of a common genetic background among random samples of animals taken from the same mass fertilization may not be generally valid. Knowledge of parentage also allowed the more accurate scoring of allozyme loci. As expected, triploids were found to be significantly more polymorphic than diploids. However, MI triploids were not significantly more polymorphic than MII triploids. MII triploid genotypes were used to estimate recombination rates between loci and their centromeres. These rates varied between 0·29 and 0·71, indicating only moderate chiasma interference.