Leaking underground storage tanks are a major source of groundwater contamination by petroleum hydrocarbons. Of the approximately 1.4 million underground tanks storing gasoline in the United States, some petroleum experts estimate that 75,000 to 100,000 are leaking (Feliciano 1984). Gasoline and other fuels contain benzene, toluene, ethylbenzene, and xylenes (collectively known as BTEX), which are hazardous compounds regulated by the U.S. Environmental Protection Agency (EPA 1977). Laboratory studies were conducted in conjunction with a field demonstration project on nitrate-mediated biorestoration of a fuel-contaminated aquifer at a U.S. Coast Guard facility in Traverse City, MI. Microcosms were prepared under either aerobic or strictly anaerobic, denitrifying conditions using core samples aseptically obtained from the aquifer. The microcosms were spiked with aromatic hydrocarbons (BTEX) and incubated as 12 C. Virtually all of the aromatic hydrocarbons, including benzene, were degraded to below detection limits within seven days under aerobic conditions, although o-xylene was somewhat more recalcitrant. Under denitrifying conditions, toluene, ethylbenzene, m-xylene, and 1,2,4-trimethylbenzene were also degraded to below detection limits, although this occurred between two to three weeks. o-Xylene was only slowly degraded and benzene was recalcitrant under denitrifying conditions. In the field demonstration project, an infiltration gallery was used to recirculate water at a rate sufficient to create a water table mound encompassing the contaminated interval. After hydraulic equilbrium was achieved nitrate and nutrients were added to the recharge water. Benzene removal occurred before nitrate addition; mass balances indicated that sufficient oxygen was recirculated to account for complete biodegradation aerobically. Based on zero-order rate processes for BTEX removal, there was good agreement between removal rates observed in the field and those in the laboratory.