Precise excision of Tn10 occurs at significantly elevated frequencies in cultures of the polA7 mutant strain of Salmonella typhimurium, is further increased in polA7 dam-1 and polA7 mutB strains and decreased in a polA7 mutH background. The numbers of precise excision events occurring in polA7 strains are also significantly increased when methionine (20 micrograms/ml or less) is present in the medium but decreased when ethionine (again, 20 micrograms/ml or less) is present. When both amino present, the outcome is about a 2-fold increase in precise excision events. The involvement of mismatch repair and methylation patterns in precise excision events is discussed.