Summary The Ord Valley of tropical Western Australia has been studied for arbovirus activity following the development of a man‐made lake of considerable size, a diversion dam and an irrigation scheme. Kununurra, the largest town in the valley, is the focus for very large populations of birds and mosquitoes. The irrigation areas have not been important as mosquito breeding areas because of the excessive use of insecticides. Lake Argyle does not support high mosquito or bird populations at present. However, this may change as the ecosystem stabilizes. The mosquito fauna of the Ord Valley is dominated by Culex annulirostris.

Summary One hundred and thirty presumptive viruses have been isolated from 485 pools made from 23,872 mosquitoes collected in the Ord River area of North‐West Australia. One hundred and eleven of the virus isolates came from pools of Culex annulirostris, the dominant mosquito species caught in the vicinity of Kununurra. Forty‐five of the viruses pathogenic for newborn mice have been further characterized‐19 as Flaviviruses, 1 Alphavirus, 9 Koongol, 1 Mapputta and 15 non‐haemagglutinating viruses of which 6 are Corriparta. Thirty‐seven isolates were from Culex annulirostris. 7 from Aedeomyia catasticta and 1 from Aedes tremuls. All Corriparta isolates were from Aedeomyia catasticta. The Flaviviruses comprised 13 Kunjin and MVE isolates.

Summary. This paper presents the results of haeniagglutination-inhibition tests carried out on sera from 441 persons, 1,080 cattle and 335 birds in the Kimberley area. Two Alphaviruses (group A) (Ross River and Sindbis) and one Flavivirus (Murray Valley Encephalitis) were used for the preparation of haemagglutinins in the haeniagglutination-inhibition tests. The tests showed a high percentage of antibody to Murray Valley Encephalitis in humans, birds and cattle, with an obvious focus of infection in the study site when cattle sera from different geographic areas were compared. These results are discussed in relation to the evolution and ecology of arboviruses in the Ord River area.INTRODUCTION.