Previous studies demonstrate that members of the strictly aerobic genus Gluconobacter produce detectable quantities of hydrogen sulfide (H₂S) when incubated in SYP medium (5% sorbitol, 1% yeast extract, and 1% peptone) containing thiosulfate. This finding is puzzling, since the microbial evolution of H₂S is characteristic of anaerobic or facultative bacteria. The goal of this research was to determine the physiological role of H₂S evolution for the gluconobacters. A methylene blue method was used to quantify the amount of H₂S evolved from cultures grown aerobically for 3 days at 28°C. Five of the six tested strains of gluconobacter evolved from 6 to 68 μg of H₂S. Strains which grew to a higher density (> 300 μg cell protein/ml ) evolved between 10 and 68 μg of H₂S. Strains which grew to a lesser extent (< 140 μg cell protein/ml ) evolved no more than 6 μg of H₂S. Uninoculated SYP medium containing 1% thiosulfate showed no evidence of H₂S evolution; however, sterile SYP medium with decreasing concentrations of yeast extract and peptone showed increasing amounts of H₂S evolved. When SYP medium was exhausted by gluconobacter growth for 72 hours, then supplemented with thiosulfate, filter sterilized, and incubated for 3 days at 28°C, these sterile solutions evolved over 400 μg of H₂S. A drop in pH. similar to that which occurs during gluconobacter growth, is not sufficient to evolve H₂S. My results to date suggest that H₂S evolution results not from gluconobacter metabolism, but rather from spontaneous decomposition of thiosulfate and the depletion of media components during growth. / Master of Science