A seroepidemiological study of the prevalence of antibodies to alphaviruses (Ross River [RRV], Sindbis [SIN] and Getah [GET] viruses) was carried out on 16 842 specimens of sera collected during 1981 and 1982 from individuals of all ages living in all health regions of New South Wales. Prevalence rates were found to be low in the major coastal cities of Sydney, Newcastle and Wollongong, and in the whole of the Tablelands. In coastal populations, the rates were highest in the far and mid North Coast zones (about 16%), and slightly lower in the south. The ratio of RRV:SIN antibodies was 62:1 in coastal populations. Antibody prevalence rates varied from 10%-20% in the Western Slopes, rising to 30%-40% in centres in the Western Plains; the RRV:SIN antibody ratio here was 11:1. The unexpectedly high infection rates, especially in the Western Plains, suggest that Ross River virus, in particular, may constitute a public health problem of hitherto unsuspected significance in New South Wales.