Serologic surveillance of populations of snowshoe hares and other vertebrate species of north-central Alberta from 1961 to 1969, revealed activity of one bacterial and eight viral agents. The most prevalent agents infecting the snowshoe hare were California encephalitis and Silverwater viruses, while in other vertebrates California encephalitis and Western equine encephalomyelitis viruses were the most common. The role of the snowshoe hare in the natural history of the agents is considered as is the effect of the agent on the hare ten-year cycle of abundance.