Cytochrome a1 is a classic cytochrome that in the 1930s had already been detected in Acetobacter strains and in the 1950s was identified as a terminal oxidase. However, recent studies did not substantiate the previous observations. We have detected a cytochrome a1-like chromophore in Acetobacter aceti, which was purified and characterized in this study. The cytochrome was solubilized from membranes of the strain with octyl beta-D-glucopyranoside and was purified by single column chromatography. The purified cytochrome exhibited a broad alpha peak around 600-610 nm, which turned to a sharp peak at 589 nm in the presence of cyanide. Carbon monoxide difference spectra of the cytochrome indicated the presence of an alpha-type cytochrome. The cytochrome contained 1 mol each of hemes b and a and probably one copper ion. These results suggest that the cytochrome purified from A. aceti is the so-called cytochrome a1, and thus the existence of the classic cytochrome has been reconfirmed. The purified enzyme consisted of four polypeptides of 55, 35, 22, and 18 kDa, and it showed a sedimentation coefficient of 6.3 S in the native form. The enzyme had a high ubiquinol oxidase activity (140-160 mumol of ubiquinol-2 oxidized per min per mg of protein). When reconstituted into proteoliposomes, the cytochrome could generate an electrochemical proton gradient during oxidation of ubiquinol. Thus, cytochrome a1 of A. aceti has been shown to be a cytochrome ba terminal oxidase capable of generating an electrochemical proton gradient concomitant with ubiquinol oxidation.