Arboviruses recently isolated in Queensland include Murray Valley encephalitis virus (MVE), isolated in 1960 and 1961 from Culex annulirostris Skuse collected near the Gulf of Carpentaria; 4 other group B viruses, 2 of which (Kunjin and Kokobera) have been shown to infect man; 3 group A viruses, 1 of which (T48) may be the cause of epidemic polyarthritis; 2 members of the new Koongol group and 2 ungrouped arboviruses. The details (time, place and mosquito species) of virus isolation and the distribution of antibody are interpreted as suggesting that 3 group B viruses (MVE, Kunjin and Kokobera) may have a similar natural history, differing from that of the group B viruses Edge Hill and Stratford. Current theories of the epidemiology of MVE and epidemic polyarthritis are discussed in relation to the present findings and the need stressed for further studies of virus survival in the dry season in north and northwest Queensland, of the extent of summer disseminations of MVE virus, of the mosquito vector and animal reservoirs of the virus that causes epidemic polyarthritis and of the pathogenesis of that disease in man.