The region required for regulation of a previously characterized arginine-regulatable promoter upstream from the argC gene in the argCAEBD-cpa-argF cluster of Bacillus subtilis was defined by integration of argC-lacZ translational fusions into the chromosome at a site distant from the arginine loci. Some sequence similarity was detected between the argC regulatory region and the well-characterized Escherichia coli arginine operators (ARG boxes). This similarity was shown to be functional in vivo in that the B. subtilis repressor regulated the E. coli arginine genes, but the E. coli repressor, even when encoded by a multicopy plasmid, could not repress the B. subtilis argC promoter. In vitro binding studies using purified repressors on DNA fragments encoding operators from both E. coli and B. subtilis demonstrated interactions by both proteins.