The mutant strain AN70 (ubiE) of Escherichia coli which is known to lack ubiquinone (Young IG et al. 1971), was analyzed for menaquinone (MK) and demethylmenaquinone (DMK) contents. In contrast to the wild-type, strain AN70 contained only DMK, but no MK. The mutant strain was able to grow with fumarate, trimethylamine N-oxide (TMAO) and dimethylsulfoxide (DMSO), but not with nitrate as electron acceptor. The membranes catalyzed anaerobic respiration with fumarate and TMAO at 69 and 74% of wild-type rates. DMSO respiration was reduced to 38% of wild-type activities and nitrate respiration was missing (less than or equal to 8% of wild-type), although the respective enzymes were present in wild-type rates. The results complement earlier findings which demonstrated a role for DMK only in TMAO respiration (Wissenbach et al. 1990). It is concluded, that DMK (in addition to MK) can serve as a redox mediator in fumarate, TMAO and to some extent in DMSO respiration, but not in nitrate respiration. In strain AN70 (ubiE) the lack of ubiquinone (Q) is due to a defect in a specific methylation step of Q biosynthesis. Synthesis of MK from DMK appears to depend on the same gene (ubiE).