In Bacillus subtilis, arginine represses its biosynthetic enzymes and activates its catabolic ones via a regulator gene ahrC. A 6.2-kb EcoRI fragment of B. subtilis chromosomal DNA that includes the ahrC gene has previously been cloned. Gene ahrC was localised in a 0.8-kb HindIII sub-fragment whose nucleotide sequence was determined. An open reading frame (ORF) was present whose translated amino acid sequence showed homology to that of the Escherichia coli arginine repressor encoded by that organism's argR gene. That this ORF corresponded to ahrC was confirmed by (i) the location of the transposon in an ahrC::Tn917 insertion mutant within the ORF; and (ii) by the appearance of an AhrC- phenotype when plasmids carrying restriction fragments lying wholly within this ORF were permitted to integrate by Campbell-type recombination into the B. subtilis chromosome. This represents the first description of a repressor in a 'housekeeping' biosynthetic system in a Bacillus, and indeed of homology between regulatory proteins for any 'housekeeping' system across such a wide taxonomic barrier among prokaryotes.