The Salmonella typhimurium leu-500 auxotrophic mutant grew when cultivated in minimal medium anaerobically, but not aerobically. This mutant carries an AT --> CG mutation in the Pribnow box of the promoter region of the leucine operon and was found to be suppressible by anaerobic conditions. Analysis of the anaerobic gases revealed that hydrogen in the anaerobic gas mixture (85% N2, 10% CO2, 5% H2) is essential for the suppression of the leu-500 mutation. When leu-500 mutant cells were incubated in the presence of the hydrogen gas, the synthetic rates for the first and last gene products of the leu-500 operon were similar to those of the wildtype cells. It was concluded that the entire leucine operon was efficiently expressed in leu-500 when the cells were grown under the hydrogen gas-containing anaerobic environment. Thus, the leu-500 promoter mutant is a model system for regulation of gene expression by a specific atmospheric environment, i.e., hydrogen gas found in the anaerobic environment.