Mosquito collections made under bridges in weld county, Colorado, from February to mid-May 1961 indicated that Culex tarsalis overwintered as nulliparous females which did not oviposit until April, and that Culiseta inornata, first found as gravid and parous females, did not produce nulliparous females until May. None of the C. tarsalis but all of the C. hibernating. No encephalitis virus was recovered from any of the mosquitoes assayed.