In order to obtain information on the occurrence of arboviruses in Western Australia, sera from white persons and Australian aborigines and from animals were subjected to antibody estimations with selected viruses as a prelude to virus isolations. The serological evidence shows the presence of group A and group B arboviruses but significant differences in antibody distribution between the two groups. Antibodies to group A viruses, particularly to the Malayan mosquito virus AMM 2354, are present in both the aboriginal and the white populations over the entire territory. Neutralizing antibody to another group A virus, AMM 2021, isolated in Malaya, is found in much lower prevalence, while antibodies to the newly isolated Queensland group A virus, MRM 39, are found only in the Kimberley area. No avian group A antibodies were detected. The prevalence of group B antibodies is high in the northern part of the State and almost non-existent in the central areas. The results indicate the presence of more than one group B virus and the absence of dengue neutralizing antibody in the Australian aborigine. A unique situation exists in central Australia, where all aboriginal sera have group A antibody but none has group B antibody.