From 1961 to 1966 the U.S. Army Medical Component, South East Asia Treaty Organization conducted extensive mosquito collecting activities throughout the Kingdom of Thailand in connection with interdisciplinary studies of mosquito-borne diseases in the country. These collections, currently being studied at the Smithsonian Institution, have revealed that Thailand is endowed with a particularly rich mosquito fauna, sharing many species with India and Indochina in its northern and central monsoon areas, and having a large Malayan element in the more southern provinces. Included among the SEATO collections is an excellent series of a unique species of the genus Culex which represents a new subgenus. The description of this taxon was prepared after examination of 29 females, 27 males, and 56 larvae, 23 of which are individual rearings with associated larval and pupal skins. In the following description, terminology and chaetotaxy of the immature stages conforms to that of Belkin (1962).