Adult Culicoides variipennis sonorensis Wirth & Jones were collected using miniature suction traps during 12 nights in October and November at a southern California dairy. Traps were baited with carbon dioxide at a constant release rate of 300, 1,000, or 3,000 ml/min or with 1.3 kg of dry ice. The release rate of CO2 from the dry ice in the laboratory declined from 1,486 ml/min during the first 2 h to 303 ml/min after 10-12 h. Nulliparous and parous females responded in a similar and positive manner to increasing CO2 release rates. An automatic interval suction trap baited with CO2 was used to assess diel host-seeking periodicity. Peak host-seeking activity occurred 0-3 h after sunset, when the trap collected up to 170 females or 103 males per hour. Average peak time of collection during the dusk period was 45 min after sunset. Although numbers were lower, parity was higher in the middle of the night. A smaller, secondary peak of activity occurred near sunrise.