In 1991-1993, the density (numbers per 10 dips) and abundance (density x flooded area) of mosquitoes were studied in 3 areas of northern Thailand with different environmental conditions. Culex tritaeniorhynchus Giles, Cx. vishnui Theobald, and Cx. gelidus Theobald were predominant among the 8 Culex species collected. Abundance of these 3 species varied significantly among the 3 areas. Type of habitat classified according to agricultural practices in rainy and dry season significantly influenced larval abundance of Cx. tritaeniorhynchus and Cx. vishnui. Seasonal variation in larval abundance was significant only in Cx. vishnui. The response of the 3 vector mosquitoes to environmental heterogeneity is discussed.