Circadian patterns of flight activity in mosquitoes can influence pathogen transmission by regulating dispersal potential of vectors and contact rates between vectors and reservoir and/or dead-end hosts. We investigated circadian activity patterns of Culex erraticus (Dyar and Knab) at a wetland field site in central Alabama, by aspirating resting adults and questing females in the morning and evening hours, respectively. Mosquitoes were aspirated at regular time intervals to determine the time of day during which peak resting site-seeking and host-seeking activities occurred. Day-to-day variation in activity patterns due to wind, humidity, and temperature was examined using stepwise linear regression. We found a distinct peak in flight activity during the morning hours (2 h before and 2 h after sunrise) for females and males of Culex erraticus, the most commonly encountered species at the site. The exact time of the peak varied from day to day, and was largely a function of temperature. A less distinct peak in activity was observed for questing females in the evening, although flights generally commenced just after sunset and peaked 30–60 min after sunset. A significant amount of day-to-day variation in the number of questing females was attributable to relative humidity. Our study demonstrates predictable patterns of circadian activity for Cx. erraticus, a suspected bridge vector of eastern equine encephalitis virus. Moreover, these patterns are modulated by environmental conditions. This information may be used to develop vector control strategies and make predictions about factors that affect the spread of mosquito-vectored pathogens.