A nitrite-oxidizing bacterium was isolated from soil of the Old Botanic Garden in Hamburg and of a cornfield near Uxmal in Yucatan (Mexico). The cells are 0.5–0.8 ×1.2–2.0 μm in size, pleomorphic, mostly pear-shaped and motile by means of one subpolar to lateral flagellum. Intracytoplasmic membranes are present as caps of flattened vesicles or membrane vesicles in the central region of the cell. The organism grows best mixotrophically with a doubling time of 10 h to 18 h. The growth rate under heterotrophic conditions is slower than under mixotrophic conditions but higher than under lithoautotrophic conditions. The DNA G+C content of the DNAs of strain X14 and strain Y is 61.2 mol% and 61.6 mol%, respectively. Both strains share 100% DNA homology. Nitrobacter winogradskyi is moderately related only to the two isolates, showing an average DNA homology value of 36%. The phenotypic and genotypic differences from N. winogradskyi support evidence that more than one species of Nitrobacter exists. For Nitrobacter X14 and Nitrobacter Y the name N. hamburgensis is proposed.