Anopheles maculatus of Thailand were distinguished morphologically into densely and sparsely scaled types and cross-mating experiments between these types revealed a substantial amount of genetic divergence. Progeny of wild females of An. maculatus collected throughout Thailand from north to south were examined morphologically with respect to adult tergal pale scales. Northern progeny broods could be separated into densely scaled (willmori) and sparsely scaled (maculatus) types, or W and M types, respectively. Using these two types of An. maculatus from Chiang Rai and Nakhon Phanom, cross-mating experiments were conducted, which resulted in sterile F1 male hybrids in one direction of cross and very low survival rate of F1 male hybrids in the reciprocal cross. However, southern broods could not be separated clearly because of intergradation of scaling. These results indicate that Thai An. maculatus involves at least 2 forms differentiated morphologically, as well as by incomplete reproductive isolation.