Twenty-three horses and three humans in Queensland, Australia, were infected with a novel member of the Paramyxoviridae family of viruses in two geographically distinct outbreaks. Two of the humans died-one died of rapid-onset respiratory illness, and the other died of encephalitis. The third infected human developed an influenza-like illness and made a complete recovery. All infected humans had close contact with sick horses. Since the two outbreaks occurred at sites 1,000 km apart and no known contact between the two groups of humans and horses occurred, extensive testing of animals and birds common to the two areas was conducted. Fruit bats (Pteropus species) were found to carry a virus identical to that found in the infected humans and horses. Although there was no contact between the infected humans and the bats, some form of close contact between the horses and bats is the likely mode of infection.