Published data indicate that spawning seasons for the Asian oyster Crassostrea ariakensis and the eastern oyster C. virginica overlap. Hybrids can form, but the larvae are not viable. If C. ariakensis is introduced into Chesapeake Bay and synchronous spawning occurs with native C. virginica, hybridization could reduce the production of viable larvae (=gamete sink). We examined the effects of gamete age, sperm concentration, and ratios of heterospecific gametes on fertilization rates and hybridization between the two species. Interspecific fertilization rates were consistently lower than intraspecific rates. Fertilization rates decayed linearly with gamete age, though intraspecific fertilization rates remained above 50% for 4–6 h, indicating that long dispersal of viable gametes is possible. Fertilization rates decayed in a log-linear manner with decreasing sperm density for intra and interspecific crosses. Fertilization rates declined significantly when sperm were (1) given a choice of eggs from each species to fertilize or (2) required to compete to fertilize eggs from a single species. Hence, a gamete sink will likely occur if these two species spawn synchronously. The magnitude of the gamete sink will depend on both gamete concentrations and the relative proportion of interspecific gametes in the water column. Furthermore, genetic analysis of individual 2-day old larvae indicated that C. virginica sperm was more likely to fertilize C. ariakensis eggs than any other interspecific cross. All else being equal, the removal of C. ariakensis eggs through this mechanism may provide C. virginica with a competitive edge.