Biological removal of organic sulfur from petroleum feedstocks mag offer an attractive alternative to conventional thermochemical treatment due to the mild operating conditions and greater reaction specificity afforded by the nature of biocatalysis. Previous investigations have either reported the desulfurization of model sulfur compounds in organic solvents or gross desulfurization of crude oil without data on which sulfur species were being removed. This study reports initial sulfur speciation data for thiophenic sulfur compounds present in crude oil which may be used as a guide both as to which species are treated by the biocatalyst investigated as well. as to where biocatalyst development is needed to improve the extent of biological desulfurization when applied to whole crudes. Biodesulfurization of two different crude oils in the 22-31 degrees API specific gravity range with total sulfur contents between 1 and 2% is demonstrated in 1-dm(3) batch stirred reactors using wild type Rhodococcus sp IGTS8, while analysis of the crudes before and after biodesulfurization did not reveal a decrease in total sulfur, GC-I;IS did reveal significant (43-99%) desulfurization of dibenzothiophenes (DBT) and substituted DBTs. Fractionation of the whole crude, followed by analysis using gas chromatography-sulfur chemiluminescence detection (GC-SCD) of the aromatic fraction of the Van Texas crude oil, demonstrated a reduction of sulfur in this fraction from 3.8% to 3.2%. This research indicates that IGTS8 may be capable of biodesulfurization of refined products such as gasoline and diesel whose predominant sulfur species are dibenzothiophenes, Further biocatalyst development would be needed for effective treatment of the spectrum of sulfur-bearing compounds present in whole crudes.