Bacteriophage T7 was challenged with either of two toxic genes expressed from plasmids. Each plasmid contained a different gene downstream of a T7 promoter; cells harboring each plasmid caused an infection by wild-type T7 to abort. T7 evolved resistance to both inhibitors by avoidance of the plasmid expression system rather than by blocking or bypassing the effects of the specific toxic gene product. Resistance was due to a combination of mutations in the T7 RNA polymerase and other genes expressed at the same time as the polymerase. Mutations mapped to sites that are unlikely to alter polymerase specificity for its cognate promoter but the basis for discrimination between phage and plasmid promoters in vivo was not resolved. A reporter assay indicated that, relative to wild-type phage, gene expression from the plasmid was diminished several-fold in cells infected by the evolved phages. A recombinant phage, derived from the original mutant but lacking a mutation in the gene for RNA polymerase, exhibited intermediate activity in the reporter assay and intermediate resistance to the toxic gene cassettes. Alterations in both RNA polymerase and a second gene are thus responsible for resistance. These findings have broad evolutionary parallels to other systems in which viral inhibition is activated by viral regulatory signals such as defective-interfering particles, and they may have mechanistic parallels to the general phenomena of position effects and gene silencing.