Aquifers contaminated with gasoline and other fuels often exhibit levels of benzene, toluene, ethylbenzene, and xylenes (BTEX) in excess of regulatory limits mandated by the U.S. Environmental Protection Agency. Addition of acetylene to microcosms simultaneously amended with nitrate and alkylbenzenes resulted in inhibition of the rate of alkylbenzene biodegradation under denitrifying conditions. Toluene, xylenes, and 1,2,4-trimethylbenzene were recalcitrant, whereas ethylbenzene was degraded at a slower rate than usual. Benzene was not degraded in either case. Addition of acetylene to microcosms preexposed to nitrate and alkylbenzenes produced similar inhibition. These data indicate that the activities of microorganisms that degrade alkylbenzenes under denitrifying conditions may be suppressed if the standard acetylene block technique is used to verify denitrifying activity.