We determined whether wing lengths ofCulex tarsalis(Coquillett) andAnopheles freeborni(Aitken) varied seasonally; whether size was associated with abundance; and if individuals of a particular size could be associated with riparian, pasture, or rice field habitats within the rice culture agroecosystem of northern California. Significantly, the size of both male and femaleCx. tarsalisincreased as the season progressed. The size of femaleAn. freebornialso increased significantly throughout the season, but the size of males did not. Abundance was not correlated with average wing length nor could any habitat be significantly associated with a particular size group. We conclude that for the species studied here, environmental effects (such as water temperature and seasonal availability of larval nutrients) are more important in determining individual size than the density-dependent processes that regulate the size of mosquitoes that breed in small containers.