We previously identified two oxidases in the membranes of bacterium Bacillus sp. FTU. One of them slowly (caa3) and the other rapidly (bo) recombines with carbon monoxide (CO) after laser flash photolysis, in this respect resembling the Escherichia coli bo- and bd-type oxidases, respectively. In the present study we found three copper atoms in the slowly CO-recombining oxidase from Bacillus sp. FTU. In the other oxidase, the copper content is very low and clearly substoichiometric. Reversed-phase chromatography revealed the presence of haems A and C in the Bacillus sp. FTU copper-containing oxidase and haems B and C in the non-copper-containing one. We thus suggest that the Bacillus sp. FTU oxidase rapidly reacting with CO previously attributed to bo-type by analogy in redox spectrum with the E. coli enzyme be redefined as bb-type oxidase.