The degree of sexual isolation among three species of mosquitoes, Aedes albopictus (Skuse), Aedes flavopictus downsi Bohart and Ingram, and Aedes riversi Bohart, occurring in Okinawajima, Ryukyu Islands was assessed by "hide-and-seek" tests and experimental crosses. The males of Ae. albopictus and Ae. f. downsi could readily discriminate conspecific females. However, under a non-choice condition, interspecific insemination took place. The cross between Ae. albopictus females and Ae. f. downsi males produced a small number of fertile hybrids. These hybrids were bred through the F_2 generation and appeared to be normal in every way. However the eggs from the reciprocal cross were completely sterile. Backcrosses of F_1 hybrids between female Ae. albopictus and male Ae. f. downsi to both parents produced partially fertile eggs, with the exception of Ae. albopictus males crossed with hybrid females. No insemination was observed in the Ae. riversi females crossed to either Ae. albopictus males or Ae. f. downsi males, as well as in the reciprocal crosses. Pre-and postmating isolation mechanisms were suggested to be operative among these 3 species.