Thirty-four strains of Asian and Pacific mosquitoes belonging to 22 species of 7 genera were compared for oral and/or parenteral susceptibility to infection with 1 or more strains of each of the 4 dengue serotypes. Surprisingly, several species of common man-biting Aedes were much more susceptible to oral infection with each of the 4 dengue serotypes than was Aedes aegypti. These species included Aedes albopictus and members of the scutellaris group of the subgenus Stegomyia found on South Pacific islands. Mosquito strains and species relatively susceptible to 1 dengue serotype usually were relatively susceptible to the others also. Almost all species of Aedes tested were uniformly susceptible to parenteral infection with the dengue viruses but, with the exception of a species of Tripteroides, species of all other genera were comparatively resistant to that mode of infection. Dengue viruses usually replicated to about the same extent in orally-infected mosquitoes as they did in parenterally-infected specimens of the same species. Seventeen species of mosquitoes of 7 genera also were tested for parenteral susceptibility to infection with Japanese encephalitis virus. With the possible exception of 2 species of Anopheles, the virus replicated to about the same degree in all species tested and achieved levels considerably higher than did any of the dengue viruses in the same mosquito strain and species held under the same conditions.