As expected, allylthiourea selectively inhibited ammonia oxidizers at a concentration of 86 μM without affecting other activities (Table 1), and 80% inhibition of ammonia oxidation was observed at a concentration of 1 μM (Fig. 1); these results are similar to results obtained previously withNitrosomonas europaea (6, 10). The complete inhibition observed after 10 min of exposure to 10 μM allylthiourea was not instantaneous (Fig. 1). Instantaneous, complete inhibition was observed at an allylthiourea concentration of 86 μM (Fig. 1).