Two field strains of Ross River virus (RRV) which differed in virulence for laboratory mice were maintained without detectable change in virulence when passaged alternately in Aedes aegypti mosquitoes and newborn mice. No biological mechanism or selection pressure was identified to explain this suppression of the usual change to higher virulence observed when RRV is passed serially in infant mice. The maintenance of initial virulence by alternating passages appears to be related to the fact that A. aegypti can be infected only if fed on mice during the period of peak viraemia and that at this time a sub-population of higher virulence may not be present in high enough infectivity to be represented in the mosquito's blood meal.