in Kununurra; indeed, occasional seroconversions have been recorded in every month of the year. Elsewhere in the Kimberley region, seroconversions occur in most years towards the end of the wet season at all sites monitored, but the overall frequency tends to be less than that observed in Kununurra, except when flooding is extensive and widespread. Until about 1990, most seroconversions in sentinel chickens in the Pilbara region were due to infections with Kunjin virus, but over the next three years seroconversions to MVE virus showed a significant increase in incidence, suggesting that virus movement from the Kimberley region may be occurring more often. Since 1993, however, Kunjin virus activity has once again become more prevalent in the Pilbara area. Mosquito collections Continuing studies in 1976 and 1977 in the Ord River area using bait traps showed that while Culex annulirostris continued to dominate the mosquito fauna of the area, other species such as Coquillettidia xanthogaster, Mansonia uniformis and Anopheles bancroftii increased in number following stabilization of the margins of Lake Kununurra and the prolific growth of aquatic plant species (Wright 1981). Studies in the West Kimberley area in 1977 in the Derby area also found that Culex annulirostris was the dominant mosquito species (Wright et al. 1981). A major advance in mosquito trapping in the north of Western Australia was the introduction of the EVS-CO light trap in 1978, which replaced the use of bait traps after 1979. This resulted in a ninefold increase in the number of mosquitoes being collected, and a significant increase in the species diversity, although Culex annulirostris remained the dominant species (Stanley 1979). Annual mosquito collections have continued to be undertaken in the Ord River area and at other sites in the Kimberley region since 1978, particularly at the end of the wet season although also at other times if unusual environmental conditions such as cyclones or early wet season flooding have occurred. With the stabilization of Lakes Argyle and Kununurra and of the area under irrigation, the results obtained have provided a clearer association between environmental conditions, mosquito numbers and virus activity (see below). Although the mosquito density, and thus the number collected, is always relatively high in the Ord River area, heavy wet season rainfall and flooding result in a significant increase in the mosquito density. In other areas of the Kimberley, a similar pattern has emerged but the increase in the mosquito density is often more marked than in the Ord River area, and the proportion of different mosquito species tends to vary considerably. Nevertheless, regardless of the study area, Culex annulirostris dominates after widespread heavy rainfall and flooding, but if the rainfall is more localized, other floodplain breeding species such as Aedes normanensis may dominate initially (e.g. Broom et al. 1992).