This study was conducted between May and October 2003 and between May and October 2004 to determine the composition of mosquito species in Istanbul. Totally 1989 adult mosquitoes, 826 (41.53%) males and 1163 (58.47%) females were collected by larvae and pupae rearing and by using CDC miniature light traps. Larvae and pupae were collected from 71 breeding sites belonging to 14 different habitats (tire, marsh, cellar, stream, temporary natural pond, permanent natural pond, septic tank, lake, pool, well, puddle, water tank, water canal and water storage cups). Identification of the species was based on morphological characters of females. As a result, eight species belonging to five genera were identified. Out of 1085 females reared from of larvae and pupae, 992 (91.4%) were Culex pipiens, 32 (2.95%) Culiseta longiareolata, 22 (2.03%) Anopheles maculipennis complex (except Anopheles sacharovi), 18 (1.66%) An. claviger, 13 (1.20%) Cs. annulata, 3 (0.28%) Aedes vexans, 3 (0.28%) Cx. hortensis and 2 (0.18%) Ochlerotatus caspius. All of the 78 females collected by using the CDC miniature light traps were Cx. pipiens. Out of 14 habitat types, Cx. pipiens, An. claviger, An. maculipennis complex (except An. sacharovi), Cs. longiareolata, Ae. vexans, Cs. annulata, Oc. caspius and Cx. hortensis was found in 14, 6, 5, 3, 2, 2, 2 and 1 habitat type, respectively. Except Cx. hortensis, all identified species are known to be capable of transmitting some pathogens to humans. It is suggested that, these species may have an important role for public health in Istanbul in the future.