The structural genes for the cyanide-insensitive terminal oxidase (CIO) of Pseudomonas aeruginosa were sequenced. The locus comprised two open reading frames, cioA and cioB, coding for gene products of 488 and 335 amino acid residues with predicted molecular masses of 54241 and 37016 Da respectively. These genes were encoded by a 2.7 kb transcript and probably comprise an operon. Upstream of a major transcriptional start site is a -10 promoter region and, approximately at nucleotides -50 and +13, there are sequences homologous to the binding site of the transcriptional regulator Anr. The deduced amino acid sequences of CioA and CioB are homologous to the cytochrome bd quinol oxidases of Escherichia coli and Azotobacter vinelandii. However, no cytochrome d-like signals were found in wild-type P. aeruginosa strains. An atypical cytochrome d-like signal was seen under low-aeration growth conditions but only in strains in which the cioAB genes were present on a high-copy-number plasmid. The appearance of these cytochrome d-like signals was not paralleled by a concomitant increase in CIO activity. These data support the hypothesis that the CIO of P. aeruginosa does not contain haem d. This raises the possibility that there is a family of bacterial quinol oxidases related to the cytochrome bd of E. coli that can differ in their haem composition from the E. coli paradigm.