In a laboratory study, ammonia (NH3) was trapped from 10 g soil units treated with 10 mg urea‐N, 10 mg urea‐N plus 50 ug N‐(n‐butyl) thiophosphoric triamide (NBPT), or 10 mg urea‐N plus 50 ug phenyl‐phosphorodiamidate (PPD). The soil was a Dothan loamy sand with pH levels adjusted to 6.0, 6.5, and 6.9 prior to N application. After 12 days, NBPT reduced NH3volatilization 95 to 97%, while PPD reduced it 19 to 30%. Although NH3loss was positively related to initial soil pH, there was no interaction between pH and urease inhibitor. In a field study, NH3was trapped in semi‐closed chambers from 134 kg N/ha surface applied to corn (Zea maysL.) 6 weeks after planting. Nine days after N application, NH3losses were 20.5, 1.5, 1.5, and 0.2 kg N/ha from urea, urea plus 0.25% NBPT, urea plus 0.50% NBPT, and ammonium nitrate, respectively. Covariance analysis showed that percent organic matter was negatively related to NHL losses. The soil properties, initial pH, CEC, and percent sand, did not vary enough to affect NH3volatilization. In conclusion, in both the laboratory and the field, NBPT exhibited strong control of NH3volatilization, and could thereby prevent significant loss of surface‐applied urea‐N to crops.