During a 1967 arbovirus survey of Utah's Great Basin. mosquitoes, collected by light traps in Cedar and adjacent Utah Valley, were pooled by species, and assayed by routine procedures in suckling mice and chick-embryo cell cultures. Viruses were identified by neutralization in suckling mice and complement-fixation tests. Culex tarsalis, Culex erythrothorax, Culiseta inornata, and Ades dorsalis constituted 91% of the total 18.878 mosquitoes collected. All 23 arbovirus isolates were from these four species. Eight Hart Park-like isolates were obtained only from C. tarsalis. The remaining 15 isolates were California encephalitis group viruses, seven from C. inornata, six from A. dorsalis. one from C. tarsalis, and one from C. erythrothorax. The best vector of California encephalitis group virus was indicated to be C. inornata by the highest total infection ratio.