The triphenyltetrazolium chloride (TTC) reduction activities of alkalophilic Bacillus sp . I-1 were higher than those of B. subtilis (Nishigami & Oshima, 1983). The component parts involved in membrane-bound respiratory chain and the mutual relationships between these components and TTC reduction activity of both strains were compared. The contents of cytochromes and quinones, and NADH dehydrogenase activities of B. sp. I-1 were higher than those of B. subtilis. In the cell-free experiment of B. sp. I-1, the reduction of TTC was only observed when NADH was used for the substrate. Further, inhibitory effects of 2-hepty1-4- hydroxyquinoline (HQNO), an inhibitor of respiratory chain, on the activity of TTC reduction were lower than those on NADH oxidase activity. Thus, NADH dehydrogenase may play an important role in TTC reduction of B, sp. I-1.