Mechanisms of gene regulation have not yet been extensively studied in thermophilic bacteria. In previous studies we showed that the Bacillus stearothermophilus argCJBD gene cluster is subject to specific repression by arginine. Here we report the cloning by colony hybridization, and characterization of the proximal part of the argC gene together with the adjacent control region of the cluster. The promoter was identified by primer extension mapping of the argC transcription startpoint: a sequence overlapping it was found to be similar to the arginine operators of B. subtilis and to a smaller extent of E. coli. Use of an argC-lacZ gene fusion revealed that the argC promoter is strongly repressed by the heterologous B. subtilis arginine repressor/activator AhrC in E. coli cells. Mobility shift and DNase I footprinting experiments revealed tight, specific and arginine-dependent binding of this operator-like sequence to purified AhrC. It is therefore very likely that in B. stearothermophilus the expression of the argCJBD operon is modulated by a repressor that is the thermophilic homologue of AhrC.