SummaryTests for VN and HI antibodies against reoviruses in the sera of quokkas of Bald Island, wallabies of the Abrolhos Islands, sheep, bats, rainbow trout, and humans of different age groups, have confirmed the apparent extraordinary ubiquity of reoviruses. HI and VN antibody to all three types of reovirus are present in a high percentage of sera of quokkas inhabiting Bald Island, As far as historical records show, these marsupials have had little contact with humans. These findings do not support the concept that man is the major reservoir of reovirus infection. Possible explanations for the widespread occurrence of reovirus “antibody” in vertebrates are considered.