Using an Escherichia coli lac deletion strain lysogenized with lambda phage carrying a metF-lacZ gene fusion (lambda Flac), in which beta-galactosidase levels are dependent on metF gene expression, cis-acting mutations were isolated that affect regulation of the Salmonella typhimurium metF gene. The mutations were located in a region previously defined as the metF operator by its similarity to the E. coli metF operator sequence. Regulation of the metF gene was examined by measuring beta-galactosidase levels in E. coli strains lysogenized with the wild-type lambda Flac phage and mutant lambda Flac phage. The results suggest that the mutations disrupt the methionine control system mediated by the metJ gene product, but not the vitamin B12 control system mediated by the metH gene product. The results also demonstrate that negative control of the metF gene by the metH gene product and vitamin B12 is dependent on a functional metJ gene product.