Diel and seasonal patterns of flight activity ofCulicoides variipennis(Coquillett),C. crepuscularisMalloch,C. hieroglyphicusMalloch, andC. palmeraeJames (Diptera: Ceratopogonidae) were studied in northeastern Colorado.Culicoides variipenniswas active Mar.–Nov., flying in the late afternoon in Mar., Apr., and Nov., throughout the diel pehod in May, June, Sept., and Oct., and during scotophase in July and Aug. Diel activity was greatest near sunset but sometimes increased near sunrise. Flight activity also increased during periods of moonlight. Flight was not observed at temp35°C.Culicoides crepusculariswas active May–Nov. Diel flight activity was greatest immediately after sunset, diminished during late scotophase, but increased occasionally near sunrise or during early photophase. Numbers ofC. crepuscularismales in flight increased during periods of moonlight.Culicoides hieroglyphicusandC. palmeraewere active Apr.–Oct. and May–June, respectively. Patterns of flight activity of both species were apparently a modal, but the greatest numbers were collected in the afternoon in June.