An ammonium-oxidizing chemolithotroph has been isolated from Alaskan waters that is different from any of the previously described species. The cells are 2.0–4.0 μm long and 1.2–2.2 μm wide. The cells contain flattened peripheral membranes that often expand, forming spherical structures near the ends of the cells. The cytomembrane structure is significantly different from any described species. The organism is an obligate halophile. The mol% G + C is 45.8, and DNA–DNA hybridization results indicate that the isolate is a new species; the name Nitrosomonas cryotolerans is proposed.