In situ biotransformation of BTEX (benzene, toluene, ethylbenzene, o-, m-, and p-xylenes) was investigated for a gasoline spill at Seal Beach Naval Weapons Station (Schroeder 1991) under methanogenic conditions in three controlled-release push-pull experiments. To create methanogenic conditions, anaerobic ground water (710 to 1365 L) was extracted from the anaerobic test zone, treated by deionization to remove nitrate and sulfate, and helium-purged to remove any traces of oxygen. Prior to release through the multiport injection/extraction well, the injection water was amended with BTEX compounds (160 to 367 μg/L) and bromide tracer. Contaminant transformation was observed in three consecutive experiments by withdrawing samples at regular intervals for periods of 73 to 159 d. BTEX removal rates were rapid for toluene and o- and m-xylenes (<30 d), and slow for benzene, ethylbenzene, and p-xylene degrading (50% removal in 60 to 90 d). Methane was formed in all cases, and the levels of soluble iron, sulfate, and nitrate were too low to account for the levels of BTEX transformation observed. The data confirm that the presence of electron acceptors (oxygen, nitrate, iron, sulfate) is not a precondition for natural attenuation to occur.