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Eight isolations of a new arbovirus, Buttonwillow virus, were made from the blood of Sylvilagus auduboni and Lepus californicus and from pools of Culicoides variipennis collected in Kern County. California. The virus had a broad antigenic relation to the Simbu group and a slight cross-relation to the California group. It is the first representative of the Simbu group to be isolated from North America. Buttonwillow virus is considered to be an arbovirus because of its isolation history, antigenic relation to other arboviruses, viremia in rabbits, biologic association with arthropod vectors, and sensitivity to sodium deoxycholate. Geographic studies of the distribution of antibodies in rabbits and hares indicate that this or a closely related virus also occurs in Northern California. Montana, Utah. Wisconsin, and Canada. Culicoides variipennis is the only naturally infected arthropod known. although infection can be established experimentally in Culex. Anopheles. and Aedes mosquitoes.