In the thermohalophilic bacterium Rhodothermus marinus, the NADH:quinone oxidoreductase (complex I) is encoded by two single genes and two operons, one of which contains the genes for five complex I subunits, nqo10-nqo14, a pterin carbinolamine dehydratase, and a putative single subunit Na+/H+ antiporter. Here we report that the latter encodes indeed a functional Na+/H+ antiporter, which is able to confer resistance to Na+, but not to Li+ to an Escherichia coli strain defective in Na+/H+ antiporters. In addition, an extensive amino acid sequence comparison with several single subunit Na+/H+ antiporters from different groups, namely NhaA, NhaB, NhaC, and NhaD, suggests that this might be the first member of a new type of Na+/H+ antiporters, which we propose to call NhaE.