Chromosome constitution was investigated in adult tetraploid Pacific oyster produced by blocking the first polar body of triploid eggs which were fertilized with haploid sperms. A high incidence of aneuploid and heteroploid mosaics were found among the offspring. Of 20 individuals identified, only 9(45%) were eutetraploid which contained 40 chromosomes; 2(10%) were aneuploid(hypotetraploid), which contained 39 and 38 chromosomes, respectively; and 9(45%) were heteroploid mosaics. One mosaic was consisted of cells containing 40 and 39 chromosomes, respectiovely(1:1 in cell number), while the other 8 were consisted of cells containing chromosomes varying between tetraploid and triploid. It was also interesting to note that 3 mosaics even contained some diploid cells with 20 chromosomes. A certain number of cells of 2 tetraploids and 8 mosaics spread with 32-37 well-scattered and some clumped chromosomes at metaphase. The percentage of aneuploid cells with chromosomes varying between triploid and tetraploid correlated significantly with that of heteroploid mosaics cells with clumping chromosomes(P<0.05). Our findings suggested that reversion existed in both tetraploid and triploid oyster and chromosome clumping may underline the chromosome elimination in tetraploid oyster. It seems that the reversing cells, at least some of them, continuously eliminate their chromosomes until the most stable diploid state is established. Chromosome constitution was investigated in adult tetraploid Pacific oyster produced by blocking the first polar body of triploid eggs which were fertilized with haploid sperms. A high incidence of aneuploid and heteroploid mosaics were found among the offspring. Of 20 individuals identified, only 9 (45% ) were eutetraploid which contained 40 chromosomes; 2 (10%) were aneuploid (hypotetraploid), which contained 39 and 38 chromosomes, respectively; and 9 (45%) were heteroploid mosaics. One mosaic was consisted of cells containing 40 and 39 chromosomes, respectiovely (1: 1 in cell number), while the other 8 were consisted of cells containing chromosomes varying between tetraploid and triploid. It was also interesting to note that 3 mosaics yet contained some diploid cells with 20 chromosomes. A certain number of cells of 2 tetraploids and 8 mosaics spread with 32-37 well-scattered and some clumped chromosomes at metaphase. The percentage of aneuploid cells with chromosomes varying between trip loid and tetraploid correlated significantly with that of heteroploid mosaics cells with clumping chromosomes (P <0.05). Our findings suggested that reversion existed in both tetraploid and triploid oyster and chromosome clumping may underline the chromosome elimination in tetraploid oyster. It seems that the reversing cells , at least some of them, continuously eliminate their chromosomes until the most stable diploid state is established.