California (CAL) group virus activity was studied in the area of Dunnville (42 50'N, 79 40W), Southern Ontario. In 1979, two of five study sites yielded CAL group virus. At on site we isolated Jamestown Canyon (JC) virus from Aedes euedes, Ae. stimulans-fitchii and unsorted Aedes spp. At the second we isolated JC virus from Ae. stimulans-fitchii and snowshoe hare (SSH) virus from Ae. stimulans-fitchii and Ae. canadensis and demonstrated seroconversions of sentinel rabbits to SSH virus. In 1981 the second site showed continuing virus activity over a 1 km transect of varying terrain including forest, scrub vegetation open lawn. Seroconversions of sentinel rabbits to SSH and trivittatus viruses were demonstrated and JC and Flanders viruses were recovered from pools of Ae. stimulans and Culex spp., respectively. Neutralizing antibodies with highest titer to SSH virus were found in 23 (5.7%) and JC virus in 2 (0.5%) of 406 Dunnville residents tested between 1979 and 1982. No clinical infections due to CAL group were documented despite increased diagnostic surveillance during this study.