Baltimore Harbor (Baltimore, MD) sediments were utilized to initiate anaerobic enrichment cultures with polycyclic aromatic hydrocarbons (PAHs) in the absence of supplementary electron acceptors. Cultures amended with naphthalene and phenanthrene exhibited sustained, transferable degradation of the PAHs. Bromoethanesulfonic acid, a selective inhibitor of methanogenesis, inhibited the degradation of 200 mu M naphthalene and phenanthrene; molecular characterization based on 16S rRNA sequences confirmed that methanogenic Archaea were eliminated, thus providing evidence that methanogenesis is involved in the degradation pathway.