Nitrification, a central process in the nitrogen cycle, converts ammonium to nitrite or nitrate. In experiments with estuarine sediment, addition of 60 and 100 mu M hydrogen sulfide (HS-) reduced nitrification by 50 and 100 percent, respectively. Aerobic incubation of ammonium-enriched sediment slurries showed that previous HS- exposure reduced nitrification for at least 24 hours; nitrification rates recovered slowly after one-time HS- exposure, Sulfide inhibition of nitrification could limit nitrogen loss through coupled nitrification-denitrification and may contribute to the previously observed difference in net nitrogen cycling between freshwater and marine sediments. This interaction could also exacerbate eutrophication in coastal environments.