Pools of mosquitoes collected in the Murray Valley in February, 1974, during an encephalitis epidemic yielded 239 isolates of 11 distinct viruses. These included 39 isolates of MVE virus, an incriminated causative agent of encephalitis in man, and 111 isolates of Kunjin virus, a probable causative agent. An additional isolate of MVE virus was recovered from the serum of a white-faced heron, Ardea novaehollandiae. The other 9 viruses comprised the Alpha-viruses Ross River and Sindbis, the Flavivirus, Edge Hill, the Bunyaviruses, Koongol and Wongal, the undefined Kowanyama virus, and 3 previously undescribed viruses. One isolate of MVE virus was recovered from a total of 180 Culex australicus, and all other isolates of all viruses were from Culex annulirostris. The combined apparent infection rate of MVE and Kunjin viruses in Cx annulirostris was 1/913. The concurrent activity of so many seemingly transient arboviruses, their wide geographical distribution and temporal considerations all weaken the widely held hypothesis that during the pre-epidemic period MVE virus is introduced into the Murray Valley by viraemic waterbirds from distant areas of tropical enzooticity.