This report provides genetic evidence that two vaccinia virus genes, A18R and G2R, both of which affect the fidelity of viral transcription in vivo, interact with each other or act on a common biochemical pathway. Previous experiments with the antipoxviral drug isatin-beta-thiosemicarbazone suggest that lethal mutation of gene G2R would compensate for mutations in gene A18R. We therefore tested the hypothesis that gene G2R is an extragenic suppressor of A18R mutations. First, we constructed a recombinant which contains both a G2R deletion mutation and an A18R temperature-sensitive mutation and found that this recombinant was viable. Second, we isolated both cold-sensitive and temperature-insensitive phenotypic revertants of A18R temperature-sensitive mutants and found in both cases that the revertants contained G2R mutations. In the case of the cold-sensitive revertants, we were able to prove that the cold-sensitive phenotype mapped to the G2R gene. Combined with the biochemical data on A18R and G2R, these results imply that the A18R and G2R genes interact with each other either directly or indirectly in a fashion which affects the fidelity of intermediate and late viral transcription.