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.

Summary A survey of mosquito populations in the Derby area of the Kimberley region, Western Australia, in March/April of 1977 yielded a total of 3,318 adult female mosquitoes. Fifteen taxa were represented, seven being new locality records for this area. Culex annulirostris was the dominant species, comprising 85·41% of the total catch. All mosquitoes collected were processed for virus isolation and thirteen strains of four (and possibly five) distinct arboviruses were obtained, all from pools of Culex annulirostris. These viruses include Murray Valley encephalitis, Ross River, Wongal, an untyped non‐haemagglutinating member of the Koongol group and a virus which reacts to polyvalent antisera against the Anopheles A and B groups. With the exception of Ross River, all these viruses had been previously isolated from the Ord River Valley, some 500 km to the north‐east. Comparison of virus isolations in the Ord Valley and Derby supports the suggestion that both sites share a common viral flora. Both also display an exceeedingly high overall isolation rate (approximately 20% for Cx. annulirostris pools). Such comparisons suggest that a number of arboviruses transmitted by Cx. annulirostris are active throughout the Kimberley region and have peak isolation rates at the end of the wet season. Further studies are needed to fully define these viral cycles.