Ross River virus, Barmah Forest virus, and Murray Valley encephalitis virus have been responsible for the majority of arbovirus infections in the Northern Territory over the past 12 years. Epidemic polyarthritis, caused by Ross River virus, was first recorded in the Northern Territory during the second World War, when troops were affected in military areas stretching from Darwin to Larrimah 400 kilometres south.[7s]. Sero prevalent surveys from 1957 [8s] to 1974 [9,10s] have suggested wide spread human exposure to Ross River virus in the Northern Territory (Doherty 1973,74). Each year from 1980 to 1989 there have been an increasing number of serologically confirmed cases of epidemic polyarthritis from widespread locations in the NT, but there has been little information on the probably vectors in the various areas. Isolations of Ross River virus from mosquitoes have been made in the Northern Territory from Darwin to Larrimah [Standfast et al 1984, Whelan and Shorthose 1982]. These isolations have been primarily from Culex annulirostris, Aedes vigilax and Aedes normanensis, with single isolation from Aedes normanensis, and Aedes vigilax. In the wet season of 1990-91, a record outbreak of epidemic polyarthritis occurred in the Northern Territory, especially in the Top End. This presented an opportunity to obtain epidemiological information, to indicate the probable vectors, and to assess the impact of mosquito control measures in various towns. Results of this outbreak are reported here.