Ubiquinone was extracted from free-living Rhizobium japonicum, grown aerobically or anaerobically, and from the symbiotic bacteroid form; it was tentatively identified as the Q-10 homologue. The ubiquinone concentration was highest in symbiotically grown R. japonicum but the ratio ubiquinone:total cytochrome was about 1•5:1 in membrane particles from organisms grown under all three conditions. The ubiquinone was reduced 75% by NADH, completely oxidized by oxygen but not oxidized by nitrate. NADH oxidase activity and nitrate reductase activity in membrane particles from organisms grown under the different conditions were similar except that nitrate reductase activity was low in aerobically grown organisms. It is concluded that ubiquinone functions in electron transport to oxygen but not to nitrate.