Evidence of genetic incompatibility observed in F1 hybrids from reciprocal crossing experiments between Anopheles nivipes and An. Philippinensis strongly suggests that these 2 nominal taxa are distinct species. Hybrid inviability of the F1 crosses was very obvious in egg eclosion rates and larval and pupal mortality. These data also suggest sex ratio distortion. Male sterility was noted in F1 hybrids from crosses in both directions, and was expressed by abnormal external genitalia, abnormal internal reproductive organs and absence of sperm.