A successful anaerobic bioaugmentation was carried out on a trichloroethene (TCE)-contaminated aquifer at Dover Air Force Base, DE, using a microbial enrichment culture capable of dechlorinating TCE to ethene. A hydraulically controlled pilot system 12 18 m was constructed 15 m below ground surface in an alluvial aquifer to introduce nutrients and substrate into the groundwater. Ambient TCE and cis-1,2-dichloroethene (cDCE) concentrations in groundwater averaged 4800 and 1200 íg/L. The pilot operated for 568 days. Results by day 269 confirmed previous laboratory work showing that dechlorination did not proceed past cDCE. By this time, most of the TCE was dechlorinated to cDCE, and cDCE was the predominant contaminant. An ethene-forming microbial enrichment culture from the Department of Energy’s Pinellas site in Largo, FL, was injected into the pilot area. After a lag period of about 90 days, vinyl chloride and ethene began to appear in wells. The injected culture survived and was transported through the pilot area. By day 509, TCE and cDCE were fully converted to ethene.