Sera from twelve species of native and five species of introduced mammals collected on the south coast of New South Wales between 1982 and 1988 were tested for antibodies to the following arboviruses: Ross River virus (621 animals tested); Barmah Forest virus (371); Gan Gan virus (337); Trubanaman virus (378). Serum neutralising antibodies to Ross River virus were found in bandicoots, wallabies, kangaroos, cattle, goat and horses; to Barmah Forest virus in kangaroo, cattle and horses; to Gan Gan virus in kangaroos, wallabies, rat, cows, horses and sheep; and to Trubanaman virus in kangaroos, wallabies, cows and horses. Titres to Ross River virus in seropositive native animal sera ranged from 32 to 1024 and those in seropositive domesticated animal sera ranged from 8 to 32 768. Prevalence of serum antibodies in macropodids, cattle and horses was: Ross River virus, 68, 19, 62%; Barmah Forest virus, 4, 26, 9%; Gan Gan virus, 44, 13, 13%; Trubanaman virus, 60, 3, 10% respectively. Evidence suggests that: (1) kangaroos and wallabies are major vertebrate hosts for Ross River virus; (2) the role of bandicoots warrants further investigation; (3) horses may be important amplifying hosts of the virus, which causes epidemic polyarthritis in man in Australia.