Arboviruses isolated and identified from mosquitoes in South Carolina (USA) are described, including new state records for eastern equine encephalitis virus (EEE), St. Louis encephalitis virus (SLE), Flanders virus, Tensaw virus (TEN), and a variant of Jamestown Canyon virus (JC). Mosquitoes were collected at 52 locations in 30 of 46 South Carolina counties beginning in June 1996, and ending in October 1998, and tested for arboviruses. Of 1,329 mosquito pools tested by virus isolation (85,806 mosquitoes representing 34 mosquito species or complexes), 15 pools were positive. Virus isolations included EEE from 1 pool each of Anopheles crucians complex and Culex erraticus; a variant of JC from 1 pool of An. crucians complex; a California serogroup virus from 1 pool of Aedes atlanticus/tormentor; TEN from 5 pools of An. crucians complex and 1 pool each of Culex salinarius and Psorophora ciliata; Flanders virus from 1 pool of Culiseta melanura; and Potosi virus from 1 pool each of Aedes vexans, Coquillettidia perturbans, and Psorophora columbiae. Of 300 mosquito pools tested by antigen-capture assay for EEE and SLE (14,303 mosquitoes representing 16 mosquito species or complexes), 21 were positive for EEE and I was positive for SLE. Positive EEE mosquito pools by antigen-capture assay included An. crucians complex (14 pools), Anopheles punctipennis (1 pool), Anopheles quadrimaculatus (1 pool), Cq. perturbans (4 pools), and Cs. melanura (1 pool). One pool of Cx. salinarius was positive for SLE by antigen-capture assay. Arbovirus-positive mosquito pools were identified from 12 South Carolina counties, all located in the Atlantic Coastal Plain, and from 4 of 8 Carolina bays surveyed.