Five mutants of Escherichia coli impaired on nitrite reduction were studied. All have lost NADH-nitrite reductase activity but have retained the capacity to synthesize all or part of their cytochrome c552. Three genes, nir C, nir D, and nir E were mapped at 26, 72.5 and 49.5 min, respectively. Another gene, nir F was tentatively localized around 52 min.