To evaluate the potential contribution of in situ biodegradation as a mechanism for natural attenuation of MTBE in surface water, surface water sediments were collected from streams and lakes at 11 sites throughout the US and the ability of the indigenous microorganisms to mineralize [U-14C] MTBE to 14CO2 under aerobic conditions was examined. Mineralization of [U-14C] MTBE to 14CO2 ranged from 15 to 66% over 50 days and did not differ significantly between sediments collected from MTBE contaminated sites and from sites with no history of MTBE exposure. The microorganisms, which inhabit the bed sediments of streams and lakes could degrade MTBE efficiently and this capability is widespread in the environment. Microbial degradation of [U-14C] MTBE was observed in surface-water-sediment microcosms under anaerobic conditions, but the efficiency and products of anaerobic MTBE biodegradation were strongly dependent on the predominant terminal electron accepting conditions. Microorganisms inhabiting the bed sediments of streams and lakes could degrade MTBE effectively under a range of anaerobic terminal electron accepting conditions. Thus, anaerobic bed sediment microbial processes also might contribute to natural attenuation of MTBE in surface water systems throughout the US.