This revision of the genus Culex, subgenus Culex of the Oriental region is based largely on the study of material from Southeast Asia and adjacent areas. More than 28,000 specimens [9,053 male, 13,075 female, 6,087 whole larvae (L)] have been examined. These included 6,105 adults with correlated pupal and/or larval skins (2,071 p, 4,034 lp) associated through individual rearings. Forty-two species are recognized and of these, 5 are new; 37 are revalidated and redescribed. The 5 new species are: luzonensis from the Philippines, selangorensis from Peninsular Malaysia, longicornis from Thailand, kinabaluensis from Sabah, Malaysia and philippinensis from the Philippines. The descriptions of 37 species are based on all stages (female, male, pupa and fourth instar larva) and those of the other 5 species: longicornis, epidesmus, diengensis, propinquus and orientalis are based only on the adults and/or associated pupae and larvae. Keys to all available stages and illustrations of every species are provided. The taxonomic treatment of the subgenus includes a revised morphological description, followed by a review of the distribution, taxonomic discussion, internal classification, bionomics and medical importance and the descriptions of individual species follow essentially the same format. The various groups, subgroups and complexes are briefly defined and characterized. New synonyms proposed here are: neolitoralis Bram (= alis Theobald); annulus Theobald (= vishnui Theobald); neovishnui Lien (= pseudovishnui Colless); fuscifurcatus Edwards and kangi Lien (= jacksoni Edwards); neomimulus Lien (= mimulus Edwards) and afridii Qutubuddin (= infula Theobald). Two previous nominal forms are resurrected from synonymy: edwardsi Barraud from barraudi Edwards and infula Theobald from bitaeniorhynchus Giles.