Interspecific crosses between the mosquitoes Aedes polynesiensis and Aedes malayensis have shown a unidirectional pattern of compatibility. Aedes polynesiensis females inseminated by Ae. malayensis males fail to produce viable offspring while the reciprocal cross is viable. In both crosses, rates of insemination are comparable to control rates. The Ae. polynesiensis females fail to lay eggs. One apparent reason for this is that the Ae. polynesiensis females have a high rate of mortality after insemination by Ae. malayensis males. Such mortality is an effective barrier to hybridization in that cross, and is a new class of isolating mechanism.