A strain of sulfate-reducing bacterium Desulfovibrio desulfuricans M6 was used to examine the degradation of model sulfur compounds found in fossil fuels, and to remove sulfur from crude oils of different origins and their products. The bacterium showed different abilities to degrade model compounds, ranging from 96% for benzothiophene to less than 10% for thiols. Degradation of some model compounds were inhibited in the presence of n-dodecane. Differences were also observed in the desulfurizations of crude oils from different origins. More sulfur was removed from the heavier fractions of petroleum than the crudes and the lighter fractions. Petroleum samples treated by the bacterium were analyzed by gas chromatography using a sulfur specific flame photometric detector, Some sulfur compounds were found to be removed completely, while others were unaffected. From these results, it was concluded that sulfur-carbon bonds in aromatic sulfur compounds are more susceptible to the reductive degradation by the bacterium than those in the aliphatic sulfur compounds, and that the low degrees of desulfurization on some petroleum samples are due to the nature of the sulfur present in the samples. It is suggested that the bacterium could be used to develop a process to desulfurize heavy fractions of petroleum.