Interspecific hybridization is a useful tool in genetic improvement of agriculture and aquaculture species. The Pacific oyster (Crassostrea gigas) and the Hong Kong oyster (Crassostrea hongkongensis) are both important aquaculture species in China. To determine whether these 2 species can hybridize and produce viable offspring, we conducted 2×2 factorial crosses between them. Asymmetry in fertilization was observed when C. hongkongensis eggs were fertilized readily by C. gigas sperm, but the reciprocal cross resulted in no fertilization. Embryos from C. hongkongensis female × C. gigas male crosses developed normally without noticeable defects, although their survival rate to D-stage was less than embryos of the two intraspecific crosses. From D-stage to metamorphosis, larvae of hybrid crosses had slower growth and a lower survival rate than that of intraspecific crosses. Nevertheless, 0.57% of hybrid D-larvae survived to spat stage at day 90. Hybrid spat had good survival (78.9%) to 1 y of age, but were significantly (P < 0.001) smaller than oysters of intraspecific crosses. Gonadal development was absent or retarded in most hybrids at 1 y of age, although some hybrids (39.2%) produced mature gametes. Our results show that hybridization between C. gigas and C. hongkongensis is possible in one direction. Some hybrids are viable, partly fertile, and can be used potentially for gene introgression between these two species.