Studies of crosses of different strains of C. p. molestus in widely separated countries have given very similar results: crosses between certain combinations may be fully fertile or partly sterile.In the Melbourne area of Australia, molestus x fatigans hybrids are common during summer and autumn; a similar situation occurs in Kansas, USA. In Victoria (Australia) cross-breeding between molestus and fatigans is common, but both retain their identity because of the existence of ecological barriers. It has been suggested that molestus represents a population with high gene frequencies for autogeny and stenogamy.Numerous crossing experiments have been made between fatigans and pipiens or molestus, for the most part successfully. Only in a few instances has partial or complete sterility occurred. A similar type of partial sterility has been observed when fatigans or molestus has been crossed with Culex globocoxitus.C. p. australicus is reproductively isolated from both molestus and fatigans.