The reproductive potential and genetics of triploidy were studied in the Pacific oyster. DNA content in sperm from triploids showed a single peak at 1.5c as determined by flow cytometry. In eggs from triploids, trivalents were the dominant form of synapsed chromosomes, although the degree of synapsis varied considerably within and among females. Some eggs went through complete synapsis and formed 10 trivalents, chromosomes; most had a mixture of 11-13 trivalents, bivalents, and univalents. Factorial matings were produced from diploid (D) and triploid (T) parent oysters, creating four crosses: DD, DT, TD, and TT (female first). Gametes from triploids were fully capable of fertilization. After fertilization, eggs from triploids went through two meioses and released two polar bodies as diploid eggs did. Karyological analyses showed that average ploidy of the resultant embryos was 2.0 n for DD, 2.46 n for DT, 2.52 n for TD, and 2.88 n for TT. Survival of fertilized eggs to metamorphosis and settlement was about 21% for DD, but considerably lower on other crosses: 0.0007% for DT, 0.0463% for TD, and 0.0085% for TT. Nine months after matings, all survivors from DT crosses were diploid. Survivors from TD crosses consisted of 33% diploids, 57% triploids, and 10% tetraploids. Survivors from the TT crosses consisted of 90% triploids, 4% diploids, and 6% mosaics. We hypothesize that differences in ploidy composition between DT and TD embryos and survivors were caused by pro-egg segregations that favor the retention, rather than loss, of extra chromosomes in the egg. The reproductive potential of triploids and evolutionary implications are discussed.