Q-Beta phage RNAs with inactivating insertion (8 base) or deletion (17 base) mutations within their replicase genes were transfected into Escherichia coli spheroplasts containing QB replicase provided in trans by a resident plasmid. Replicase-defective (Rep~) Q3 phage produced by these spheroplasts were unable to form plaques on cells lacking this plasmid. When individual Rep~ phage were isolated and grown to high titer in cells containing plasmid derived Q3 replicase, revertant Q3 phage (Rep'), with the original mutation (insertion or deletion) repaired, were obtained at a frequency of ca. 1 x 108. RNA recombination via a "template switching" mechanism involving Q3 replicase, the mutant phage genome, and the plasmid-derived replicase mRNA was shown to be the primary means by which these mutant phages reverted to wild type.