We have cloned and sequenced a gene, pds, from the cyanobacterium Synechococcus PCC7942 that is responsible for resistance to the bleaching herbicide norflurazon. A point mutation in that gene, leading to an amino acid substitution from valine to glycine in its polypeptide product, was found to confer this resistance. Previous studies with herbicide-resistant mutants have indicated that this gene encodes phytoene desaturase (PDS), a key enzyme in the biosynthesis of carotenoids. A short amino acid sequence that is homologous to conserved motifs in the binding sites for NAD(H) and NADP(H) was identified in PDS, suggesting the involvement of these dinucleotides as cofactors in phytoene desaturation.