A laboratory column study was set up to evaluate changes in contaminant distribution and sediment toxicity following nitrate-based bioremediation and to correlate toxicity reduction with loss of fuel components. Glass columns were packed with sediment from an aquifer that had been contaminated with JP-4 jet fuel and were remediated using feed solution containing 20 mg/L NO 3 -N. Column influents and effluents were monitored for BTEXTMB (benzene, toluene, ethylbenzene, xylenes, trimethylbenzenes), electron acceptors, nutrients, and dissolved gases. Duplicate columns were sacrificed after 1, 4, and 7 months, and core material was analyzed for chemical constituents. In addition, core material was evaluated for toxicity using FETAX, a developmental toxicity test employing frog embryos. After 1 month of operation, total mass of BTEXTMB dropped from 51.8 Ϯ 7.3 mg to 29.8 Ϯ 2.9 mg (42% reduction) in the column sediments, with 1.45 Ϯ 0.06 mg eluting in the column effluents and 20.6 Ϯ 3.0 mg being unaccountable, presumably due to biodegradation. Based on stoichiometry of denitrification, nitrate consumption and nitrite and nitrous oxide production were sufficient to account for the observed loss. In contrast, JP-4 levels only dropped from 2,070 Ϯ 260 mg to 1,750 Ϯ 22 mg (15% reduction). Despite the similar distribution of contaminants in the two columns of this first column pair, FETAX mortality remained unchanged at 93.3% in the first column and dropped from 93.3% to 11.7% in the second. Toxicity reduction could not be therefore directly attributed to either BTEXTMB or JP-4 levels. After 7 months, total mass reductions were 93.1 Ϯ 1.1% and 35 Ϯ 1.6% for BTEXTMB and JP-4, respectively, and FETAX mortality and malformation had declined to less than 10%. These data show that closure standards requiring complete removal of residual hydrocarbon may overestimate the time required to counteract sediment toxicity, and that nitrate-based bioremediation may be a viable treatment alternative for fuelcontaminated aquifers.