Effects of heterotrophic contaminants on H//2S(g) oxidation by T. denitrificans are reported. Heterotrophic contamination appears to have a negligible effect on the growth of T. denitrificans on H//2S. If mixed cultures (T. denitrificans and heterotrophs) can be used for microbial gas desulfurization the process will be simplified by removing the requirement of aseptic operation of the reactor. Heterotrophs may have a positive effect on the process by scavenging organic waste products potentially inhibitory to T. denitrificans.