The biological treatability of subsurface soil contaminated with manufactured gas plant (MGP) waste was evaluated. Mineralization assays incorporating 14C-phenanthrene were used to evaluate the biotransformation potential of indigenous microorganisms at the site. Multi-phase laboratory microcosms were used to evaluate the interphase transfer potential and chemical mass distribution of phenanthrene. The Microtox™ bioassay was used to evaluate detoxification trends at the site. Mineralization results indicated that indigenous microorganisms at the site were capable of transforming phenanthrene, a component of coal-tar creosote. Results also indicated that spiked 14C-phenanthrene mineralization was influenced by nutrient addition and by the amount of contamination. The chemical mass distribution of 14C-phenanthrene indicated that volatilization may be an important transport mechanism for chemicals residing in, or migrating to the vadose zone of soil. Following removal of the coal-tar waste source at the site, the toxicity of water soluble extracts of the site soil decreased to a non-toxic response based upon Microtox™ assay results. Parent compound concentrations at the site also decreased with time subsequent to source removal. Results of this study indicate that natural in situ bioremediation may be an important treatment process at a former manufactured gas plant waste site in New York.