SummaryRoss River virus strains recovered from north Queensland and the central coast of New South Wales can be clearly discriminated by the use of a short incubation haemagglutination inhibition test. Antigenic homology of strains within these regions and heterology between the regions seem unaffected by time, passage history, adaptation to laboratory mice and the nature of the original source material yielding the strains. It is concluded that the two antigenic types are enzootic to their respective regions and that they have evolved in isolation from a common ancestral virus. A short incubation complement fixation test was found to be more sensitive than the haemagglutination inhibition test in attempts to characterize the virus type inducing convalescent antibody in humans and horses.