Chlorate-resistant mutants were generated by random insertion of the transposon Tn5 into genomic DNA of Pseudomonas stutzeri ZoBell strain and selected for loss of nitrate respiration (Nar^- phenotype). The mutants were differentiated by restriction- fragment analysis, by assaying for nitrate assimilation and for molybdenum cofactor activity, and by the amount of respiratory nitrate reductase. Two mutants, lacking both nitrate respiration and nitrate assimilation, overproduced an inactive nitrate reductase but synthesized in the presence of nitrate only a reduced amount of respiratory nitrite reductase (cytochrome cd₁). Expression of cytochrome cd₁ in these mutants was specifically induced by nitrite, suggesting a sensor system for this substrate.