Rates of biodegradation were measured in a petroleum hydrocarbon-contaminated aquifer using a combination of field and laboratory methods. These methods are based on tracking concentration changes of substrates (both electron donors and acceptors) or final products of microbial metabolism over time. Ground water at the study site (Hanahan, South Carolina) is anoxic, and sulfate reduction is the predominant terminal electron accepting process. Laboratory studies conducted with sediment cored from the site showed that ¹⁴C-toluene was mineralized to ¹⁴CO₂ with a first-order degradation rate constant (k_tol) of -0.01 d⁻¹ under sulfate-reducing conditions. Under nitrate-amended, Fe(III)-amended, or nonamended (methanogenic) conditions, toluene was not significantly mineralized. ¹⁴C-Benzene was degraded at low but measurable rates (k_ben= -0.003 d⁻¹) under sulfate-reducing conditions whereas degradation under methanogenic conditions was negligible. These results illustrate the extreme sensitivity of laboratory-measured biodegradation rates to terminal electron-accepting conditions, and show the necessity of carefully matching experimental conditions to in situ conditions. Concentration decreases of toluene along aquifer flowpaths, when the uncertainty of ground-water flow velocities was considered, indicated k_tol values ranging from -0.0075 to -0.03 d⁻¹. Concentration decreases of sulfate and concentration increases of dissolved inorganic carbon (DIC), when normalized for assumed stoichiometric oxidation of toluene coupled to sulfate reduction, yielded a k_so4 range of -0.005 to -0.02 d⁻¹, and a k_DIC value range of +0.00075 to +0.003 d⁻¹. Because both laboratory and field methods have numerous sources of uncertainty, a combination of these methods is the most appropriate procedure for evaluating biodegradation rate constants in contaminated ground-water systems.