Colorless sulfur-oxidizing bacteria are ubiquitous in Indian waters and have the ability to oxidize sulfide under anaerobic conditions. These bacteria can not only mediate the sulfur cycle oxidatively but also the nitrogen cycle reductively without drastic change in the pH. The sulfide-oxidizing potential of some of these isolates has been monitored under different environmental conditions. Initial screening at 16 ppm (1 ppm = 1 mg of S sup(2-) S/liter in medium) showed low oxidation ability. Subsequently, the percentage oxidation was higher at 45 ppm but decreased at 64 ppm. Iron had a variable effect on sulfide oxidation. The viability of the cells and efficiency of oxidation increased in the presence of nitrate. An increase in temperature beyond 50 degrees C decreased cell viability and consequently sulfide oxidation was negligible. Cell viability as estimated by the colony-forming ability decreased 100-fold after the exposure to 100 kg/cm sup(2) pressure for 1 h. Both viability and sulfide oxidation, however, improved with prolonged exposure. The present study discusses the effect of sulfide, temperature, and pressure on the culturability of colorless sulfur-oxidizing bacteria and their possible adaptation and use in the bioremediaton of oil wells polluted by sulfide production