Eighteen strains of reovirus type 3 were isolated from mice and tissue cultures inoculated with suspensions of mosquitoes caught in the southeastern part of the main island of the Fiji group between 17 April and 12 June 1967. Three strains were isolated originally from rhesus-monkey kidney cells and suckling mice and reisolated from human-embryo kidney cells and suckling mice. Seven further strains were reisolated from suckling mice. Strains of virus isolated from mosquitoes were inoculated into laboratory colonies of Aedes australis and Culex quinquefasciatus and into wild-caught Culiseta tonnoiri. Evidence was obtained of prolonged survival but not of multiplication in these species. C. quinquefasciatus fed on viremic suckling mice were able to transmit to further suckling mice for up to 4 days after the infective feed, but no longer. This is interpreted as mechanical transmission of a highly resistant virus that probably remained active while contaminating the mouthparts of the insects.