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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.

Received for publication May 1, 1957. This work was supported by U. S. Army Chemical Corps contract, No. DA-18-064-CML2639, with the University of Utah. was obtained from Dr. George C. Wright, Fort Detrick, Frederick, Maryland. Upon arrival the suspension was diluted 1:100 in buffered saline (pH 7.0; 0.1M phosphate buffer) and stored at 4 C. From this stock suspension the "challenge" dilutions were prepared in 0.85% sodium chloride. Challenge injections.?Each species was divided into five groups of eight animals each, except where an inadequate number of animals necessitated the use of fewer groups and/or fewer animals per group. The spore suspension was injected intracutaneously into a region on the abdomen from which the fur had been plucked just prior to inoculation. In each instance a tuberculin syringe with a 27 gauge needle was used to administer the inoculum. The animals of