Heteroduplex DNA molecules that were wild type or contained combinations of amber, missense, and temperature-sensitive mutations were prepared from bacteriophage T7. These DNA molecules were then encapsulated in in vitro packaging reactions so as to produce infective T7 phage. The genotypes of the phage were examined to determine the degree to which mismatched base pairs in the heteroduplex had been corrected. The data show that conversion of the mismatches took place either during in vitro packaging or immediately after infection of either an Escherichia coli or Shigella sonnei host. The mode of mismatch conversion observed in these experiments was independent of the host mutH, mutL, mutS, and uvrD genes. There was no significant amount of discrimination between markers on either of the two complementary strands. The observed frequency of conversion of a mismatch depended on the genetic marker being monitored and on experimental conditions but was generally in the range between 5 and 30%.