For the past two years, the ESTCP program has been sponsoring a full-scale MTBE plume containment demonstration at the Naval Base Ventura County, Port Hueneme, CA. This passive flow-through biobarrier system has been designed to degrade MTBE and other dissolved hydrocarbons leaving the down-gradient edge of a residual gasoline-impacted source zone. This site is somewhat unique in that the dissolved MTBE plume is approximately 500 ft wide and about a mile long. Dissolved MTBE concentrations upgradient of the biobarrier are as great as 20 mg/L, and BTEX compounds are also present at mg/L concentrations in the central core of the plume. This biobarrier design incorporates combinations of aerobic biostimulation and bioaugmentation. Data obtained during the first 18 months of operation will be presented.Currently, MTBE and BTEX concentrations at the downgradient edge are being reduced to >0.005 mg/L (non-detectable levels). In addition to the performance data, the design and costs of this system are presented in comparison with a pump and treat system currently operating at a different location in the same dissolved plume.