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The level of reproductive isolation between the closely related Drosophila americana and D. novamexicana has been examined. Egg hatch rates in heterospecific crosses indicate that the reduction in hatch rate is due to prezygotic incompatibilities between heterospecific individuals. The two species exhibit an asymmetric level of postmating-prezygotic incompatibility that is caused by an incapacitation and/or loss of sperm from the heterospecific female's reproductive tract. Measurements of this phenomenon between iso-female lines within D. americana and a D. americana iso-female line suggests that the phenomenon is species wide and results from rapid divergence in male-female postmating-prezygotic reproductive compatibility between allopatric populations.