The response of male Culicoides variipennis sonorensis Wirth & Jones to carbon dioxide (CO2) was examined at a dairy in the Chino Basin of southern California. Males were collected using CDC-type suction traps (no light) during 6 evenings in September and October 1995 when C. v. sonorensis were abundant. Traps were baited with 1.8 kg of dry ice, 200-350 nulliparous females, or nothing. Greater than 8 times as many males were captured in CO2-baited suction traps than in either female-baited or unbaited traps. Female-baited and unbaited traps did not differ in the number of males captured. Observations of mating behavior near and on a host calf were made at a nearby dairy. Males swarmed 1-2 m downwind (east) of a restrained calf and 0.3-1.0 m above ground level. Males were also observed coupled with blood-feeding females on the calf venter (especially umbilicus and teats). Virgin female C. v. sonorensis were captured in CO2-baited suction traps and by aspiration while they were engorging on the venter of a tethered calf. There appear to be at least two mating strategies in this species: mating presumably may occur near hosts in male swarms as well as stenogamously on the venter of the host. These mating strategies may serve as prezygotic reproductive isolating mechanisms between closely related and sometimes sympatric members of the C. variipennis complex.