Summary Ammonia oxidation, as the first step in the nitrification process, plays a central role in the global cycling of nitrogen. Although bacteria are traditionally considered to be responsible for ammonia oxidation, a role for archaea has been suggested by data from metagenomic studies and by the isolation of a marine, autotrophic, ammonia‐oxidizing, non‐thermophilic crenarchaeon. Evidence for ammonia oxidation by non‐therm...