The feasibility of biologically degrading methyl tert-butyl ether (MTBE)-contaminated groundwater is dependent on the ability to degrade MTBE and its byproducts in the presence of other gasoline contaminants. This study investigates a mixed culture degrading both MTBE and benzene-toluene-ethylbenzene-xylene (BTEX) in a continuous-flow reactor with a biomass retention system (porous pot) at varying hydraulic retention times (HRTs). The reactor degraded both MTBE and BTEX simultaneously to less than the U.S. Environmental Protection Agency's drinking water standards and recommendations at an HRT of 3.76 days. Methyl tert-butyl ether was degraded from 75 mg/L to less than 1 microg/L and each BTEX compound was degraded from 17 mg/L to less than 1 microg/L. Effluent concentrations of MTBE and BTEX increased as the HRT was decreased from 3.76 to 0.52 days, but remained lower than drinking water limits. The highest MTBE effluent concentration was at 0.52 days and continued to average less than 10 microg/L. Although tert-butyl alcohol (TBA) is commonly sited as a degradation byproduct of MTBE, it remained lower than 1 microg/L throughout all reactor operation periods. The MTBE and BTEX observed yield coefficients for the reactor ranged from 0.11 to 0.16 mg biomass/mg MTBE and BTEX, with the rate increasing with increased HRT. Degradation rates from the reactor were compared to results from batch studies with the same culture on MTBE and TBA alone and in the presence of BTEX. Results of batch studies showed that BTEX was not required for this culture to degrade MTBE. The presence of BTEX only had an effect on the rate of MTBE degradation at the highest concentration, while BTEX significantly delayed the degradation of TBA at all concentrations. Batch studies also showed that the rate of degradation of TBA was higher with and without BTEX than the rate of degradation of MTBE, explaining the lack of high levels of TBA in the reactor.