This study addresses the potential problem of alphavirus infection associated with recreational use of the Ross River reservoir in north Queensland, Australia. From 1990 to 1993, 51,497 adult female mosquitoes were collected mainly by CO2-supplemented light traps. Four localities within the reservoir were considered and compared with mosquitoes collected during 1991 from 2 public localities around Townsville City. Ten isolates of Ross River virus, one of Barmah Forest virus, and 2 of Sindbis virus were recovered from Aedes normanensis, Anopheles amictus, and Culex annulirostris. All isolates were collected during the wet seasons of 1991 and 1992 using an enzyme immunoassay and cross-checked using a polymerase chain reaction assay Estimation of relative hazard was based on total mosquito abundance, prevalence of vector species, and on mosquito infection rates. Based on 1990-93 data, it was concluded that the Big Bay area of the Ross River dam, currently being considered as a primary recreational locality, was relatively safer than Antill Creek, Ross River, and Toonpan and presented no greater hazard than localities around urban Townsville, away from the reservoir. However, because of the changing ecology of the reservoir and lack of a full understanding of annual alphavirus activity, periodic surveillance is recommended.