Differences in larval habitats cause variation in the size of Aedes aegypti (L.) adults. We suspected that such size variation was related to the ease with which the mosquitoes could be infected with dengue virus. Using a rearing procedure that produced three distinct size classes of mosquitoes, we determined the percentage of mosquitoes that developed disseminated dengue-2 infection following oral feeding with a suspension containing 3.3 x 10(7) plaque-forming units/ml. Mosquitoes were reared from eggs deposited by females captured in either of two villages in Chachoengsao Province or in Bangkok, Thailand. More of the larger mosquitoes (10.7%) were infected than the medium (5.6%) or small (5.7%) mosquitoes. Mosquitoes from Bangkok were less easily infected (5.0%) than mosquitoes from either of the two villages (8.5% and 10.7%). These results suggest that quantitative risk assessment of dengue transmission may be very difficult unless inoculation rate is measured directly. Also, control procedures that reduce density of larvae in individual containers may exacerbate dengue transmission by creating larger mosquitoes that are more easily infected.