A survey of adult mosquitoes was done during 1986–1987 in an area undergoing irrigation development in the Mahaweli Project of Sri Lanka. The study encompassed two 12-mo phases of settlement and irrigation, respectively. Diurnal human bait catches yielded 1,427 female mosquitoes in 27 species. Nocturnal collections (human bait, CDC light traps, indoor resting catches) produced a total of 30,491 females in 67 species. Comparisons with an earlier survey in the predevelopment forest showed that the diurnal human biting abundance of Aedes albopictus (Skuse) remained stable, whereas the abundance of Ae. novalbopictus Barraud and Ae. w-albus Theobald declined drastically after development. Man-mosquito contact increased substantially for nocturnally active Anopheles annularis van der Wulp, An. subpictus Grassi, Mansonia uniformis (Theobald), Culex tritaeniorhynchus Giles, and Cx. quinquefasciatus Say and decreased for Ma. annulifera (Theobald) and Cx. gelidus Theobald. A small increase occurred in An. culicifacies Giles, An. vagus Donitz, An. varuna Iyengar, Cx. fuscocephala Theobald, and Cx. whitmorei (Giles); Cx. pseudovishnui Colless decreased. The effect of changes in mosquito abundance on the increased transmission of malaria and Japanese encephalitis is discussed.