The relationship between the California encephalitis (CEV) group of arboviruses and a snowshoe hare population near Rochester (54N, 113W), Alberta has been studied since 1961. The neutralizing antibody prevalence to the CEV group of viruses in adult hares was high, 58% to 95%, during the 6 years of snowshoe hare population decline, when the population dropped from over 600 hares per square mile to 3 hares per square mile. In the 2 years of population recovery thus far observed, the antibody prevalence has been low, 0% to 43%. The prevalence rates in juveniles have been lower than in the adults throughout the study. At least 2 strains of the virus group are present and a total of 7 isolations from Aedes mosquitoes and 1 from a snowshoe hare have been made, representing the northern most known limit of the CEV froup in North America. Experimental viremia studies with the Montana snowshoe hare strain of virus resulted in titers up to 10⁴ mouse LD₅₀ per ml. of whole blood and viremia lasted up to 3 days with an increase in length of viremia with age.