The bluetongue viruses are transmitted to ruminants in North America byCulicoides variipennis. US annual losses of approximately $125 million are due to restrictions on the movement of livestock and germplasm to bluetongue-free countries. Bluetongue is the most economically important arthropod-borne animal disease in the United States. Bluetongue is absent in the northeastern United States because of the inefficient vector ability there ofC. variipennis for bluetongue. The vector of bluetongue virus elsewhere in the United States isC. variipennis sonorensis. The threeC. variipennis subspecies differ in vector competence for bluetongue virus in the laboratory. UnderstandingC. variipennis genetic variation controlling bluetongue transmission will help identify geographic regions at risk for bluetongue and provide opportunities to prevent virus transmission. Information onC. variipennis and bluetongue epidemiology will improve trade and provide information to protect US livestock from domestic and foreign arthropod-borne pathogens.