Alterations of chromosome number have been observed in the somatic tissue of Crassostrea gigas diploids and artificial polyploids. Tetraploid oysters with abnormal chromosome numbers in some or all of their tissue are considered undesirable as parents either for triploids (produced via a cross with diploids and of aquacultural interest) or for tetraploid breeding. Aneuploid tetraploid oysters may confer the tendency to lose chromosomes and to revert to lower ploidy levels to their offspring. More directly, their offspring could have a lower ploidy because of potential links between somatic and gametic chromosome loss. The present study evaluated the phenomenon in six bi-parental tetraploid families bred from parents of differing somatic ploidy quality. The offspring were assessed over a year using chromosome counts. Differences between families and parental influence on chromosome loss were evaluated. This showed that chromosome loss occurred at high frequency in all families and that families differed in their composition of ploidy types. Triploidy was observed in four out of the six families. Comparison of data collected at 4 months and I year showed no worsening on this time-scale. The incidence of chromosome loss among families suggests a genetic basis to the phenomenon, although a direct relationship between the ploidy quality of the parents and that of the offspring was not observed. The origins and evolution of chromosome loss in polyploid oysters and the implications for breeding are discussed.