The reactivity between different cytochromes c purified from Pseudomonas aeruginosa cells grown aerobically in the absence of nitrate and isolated cytochromes co and baa(3) was determined. The P. aeruginosa cytochrome co reacted most rapidly with the membrane-bound cytochrome c-551 among three c-type cytochromes analyzed, whereas the cytochrome baa(3) reacted best with the membrane-bound cytochrome c-555. The results indicated that two terminal electron transfer systems are present in aerobic P. aeruginosa: one contains the cytochrome c-551 and cytochrome co, and the other contains the cytochrome c-555 and cytochrome baa(3).