To verify the genetic proximity of Anopheles sinensis strains in Korea, China, Japan, and Thailand, we established 5 iso-female lines, and investigated their genetic relationships by analyzing crossing tests and comparing the DNA sequences of rDNA internal transcribed spacer 2 (ITS2) and mitochondrial cytochrome c oxidase subunit 2 (CO2). The results of reciprocal and F₁-hybrid crosses between Thailand Form B (X, Y2) with Japanese Form A (X, Y1), Japanese Form B with Korean Form B, and Korean Form B with Chinese Form B indicated that these strains were genetically compatible, providing viable progenies and completely synaptic polytene chromosomes. The sequences of ITS2 and CO2 among 10 Asian An. sinensis strains were almost identical to each other, and the sequence variation was very low (<0.6% in both regions). Based on the above results, it was confidently concluded that An. sinensis is a single cosmopolitan species even though its geographical distribution ranges are comparatively wide (>2,000 mi) and that the malaria-vector abilities of the various strains may vary depending on their geographical origins.