Independently isolated, spontaneous pseudorevertants of temperature-sensitive (ts) mutants of reovirus type 3 have previously been genetically characterized (R. F. Ramig and B. N. Fields, 1979, Virology 92, 155-167). Eighteen of these pseudorevertants were backcrossed to wild-type reovirus type 1 and reassortant progeny expressing the parental ts phenotype were selected. Analysis of segregation of genome segments in the reassortant, parental ts, progeny clones allowed the determination of the genome segment bearing the suppressor mutation of four pseudorevertants. The suppressor of tsA(201) phenotype mapped to segment S4 in the pseudorevertants RtsA(201)101 and RtsA(201)121 and to segment L3 in pseudorevertant RtsA(201)122. The suppressor of tsB(352) phenotype mapped to segment S1 in the pseudorevertant RtsB(352)b. In two other pseudorevertants the suppressor could not be mapped to a single genome segment due to the small number of progeny clones examined. These genetic results indirectly support the "compensating protein interactions" hypothesis for the mechanism of suppression.