Three strains of Anopheles sinensis and Ae. sineroides respectively collected in Japan north to south were used for the present hybridization experiments to determine genetic affinities among strains of sineroides and between sinensis and sineroides. Comparing the F, hybrids and those of their parent species for the control, there exists no difference in the number of ova laid and their percent hatches. The F, hybrids salivary chromosomes which showed complete asynapsis verify the observation that the two entities differ chromosomally by band differences formed in their paternal entities at the telomer in region of the x and the pericentric area of 3R of their chromosomes. The low viability of the F, hybrids with high larval and pupal mortalities, and the fact that only a few F, hybrids were obtained suggest there exists reproductive isolation between both entities. The data presented suggest that An. sineroides are two separate species.