1. By difference spectrophotometry the cytochrome pattern of two strains of Rhizobium leguminosarum — effective and partly-effective — was studied in pure culture and in bacteroids isolated from legume root nodules1. 2. In pure culture both strains contained cytochromes c, b and a-a3 as well as cytochrome a2 which appeared under semi-anaerobic conditions, especially in the effective strain. In Rhizobia cytochrome a2 was detected for the first time. 3. Bacteroids of the effective strain contained cytochromes c, b and some CO-reactive pigments which remained incompletely identified. Bacteroids of the partly-effective strain contained cytochrome a (a = peak at 597 nm at − 196°) which was absent from bacteroids of the effective strain. 4. The data obtained show that the low O2 concentration within nodules does not seem to be the only factor responsible for the absence of cytochromes a from N2-fixing bacteroids.