The biodegradation of two crude oils by microorganisms from an anoxic aquifer previously contaminated by natural gas condensate was examined under methanogenic and sulfate-reducing conditions. Artificially weathered Alaska North Slope crude oil greatly stimulated both methanogenesis and sulfate reduction. Gas chromatographic analysis revealed the entire n-alkane fraction of this oil (C13-C34) was consumed under both conditions. Naphthalene, 2-methylnaphthalene, and 2-ethylnaphthalene were also biodegraded but only in the presence of sulfate. Alba crude oil, which is naturally depleted in n-alkanes, resulted in a relatively modest stimulation of methanogenesis and sulfate reduction. Polycyclic aromatic hydrocarbon biodegradation was similar to that found for the Alaska North Slope crude oil, but a broader range of compounds was metabolized, including 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene in the presence of sulfate. These results indicate that n-alkanes are relatively labile, and their biodegradation in terrestrial environments is not necessarily limited by electron acceptor availability. Polycyclic aromatic hydrocarbons are relatively more recalcitrant, and the biodegradation of these substrates appeared to be sulfate-dependent and homologue-specific. This information should be useful for assessing the limits of in situ crude oil biodegradation in terrestrial environments and for making decisions regarding risk-based corrective actions.