A mutant of the Sindbis virus SV-S was found to interfere with the regular course of infection by the wild strain of the virus SV-W in A. aegypti mosquitoes and in suckling mice. In mosquitoes, this result was manifested by a reduced titer of SV-W in the presence of SV-S and by a failure of the mosquitoes to transmit SV-W. In the brains of suckling mice, in the presence of SV-S, the growth of sc inoculated SV-W was suppressed, and as a result, the usually lethal course of infection by this virus was converted into a nonlethal one.