Effective and economical control of adult vector and pest mosquitoes requires knowledge of their seasonal abundance and host-seeking activity patterns. We conducted research in 2006–2007 to study these variables for Culex tarsalis, Aedes vexans, Ae. melanimon, and Ae. dorsalis in Larimer County, CO. Mosquitoes were collected with traps that segregated catches in 7 consecutive 2-h intervals initiating at 1730 h at 4 sites. Seasonal abundance varied for all species by site and year. Time of host-seeking activity was consistent for all species by site and year. Culex tarsalis counts were significantly higher 1.2–4.5 h after sunset than during the preceding time intervals. Maximum host-seeking activity of the 3 Aedes species occurred from 0.8 h before sunset to 6.5 h after. Host seeking by all species continued throughout the night. For optimal control of Cx. tarsalis adulticide application should start approximately 1 h after sunset, and control of Aedes species should begin soon after sunset, and for all species applications can continue throughout most of the night.