Surveys to find mosquito pathogens in Aedes aegypti and Culex quinquefasciatus (=fatigans) were conducted at 20 localities in Thailand. Specimens of a few additional mosquito species, collected incidentally, were also examined. Twenty-two mosquito host-mosquito pathogen associations were found, comprising 9 species of mosquitoes and possibly 17 pathogen species. Pathogens found included a helicosporidan in Ae. aegypti and Cx. quinquefasciatus; 3 microsporidia in Ae. aegypti; 1 microsporidian each in Cx. quinquefasciatus and Anopheles vagus; 1 or possibly 2 microsporidia in Culex (Lutzia) sp; eugregarines in Ae. aegypti, Ae. albopictus, Ae. chrysolineatus, and Armigeres subalbatus; a flagellate in Toxorhynchites splendens, Coelomomyces sp. in Cx. minor; and a highly lethal septicemia in Ae. aegypti, Cx quinquefasciatus, An. vagus, and Ar. subalbatus. A gram positive, spore-forming bacillus was found in Ae. aegypti, Ae. albopictus, and Ar. subalbatus, but its pathogenicity was not investigated.