The effects of C(2)H(2) metabolism on N(2)O production were examined in soil slurries. Enrichment of C(2)H(2) consumption activity occurred only in aerobic incubations. Rapid disappearance of subsequent C(2)H(2) additions, stimulation of CO(2) production, and most-probable-number enumerations of C(2)H(2) utilizers indicated enrichment of the population responsible. During C(2)H(2) consumption in slurries incubated statically under air, maximal rates of N(2)O evolution were 19 times higher than those in anaerobic incubations. After 20 days of enrichment with C(2)H(2), the production of N(2)O by slurries supplemented with C(2)H(2) and nitrate was 10 times higher than that in the unenriched controls. A Nocardia- or Arthrobacter-like bacterium was isolated that grew on C(2)H(2) but did not denitrify. The behavior of soil inoculated with this bacterium became similar to that of C(2)H(2)-enriched soil incubated aerobically. Ethanol, acetate, and acetaldehyde were identified in enrichment experiments, and denitrification in soil slurries was stimulated by addition of the supernatant from a pure culture grown on mineral medium with C(2)H(2). These results indicate that denitrification can be stimulated by the actions of an aerobic, nondenitrifying C(2)H(2)-metabolizing population. Utilization of intermediate metabolites by denitrifiers and enhanced O(2) consumption are two possible mechanisms for this stimulation.