An attempt has been made to find out which of the two terminal oxidases, the d-type or the o.type, operates as a Na ~ pump in Escherk, hia coli grown at low zl, aa* conditions. For this purpose, mutants lacking either d or o oxidase have been stadi~. It is shown that a d'.o" mutant grows slowly or does not grow at all under low zlBH* conditions (alkaline or protonophore-containing growth media were used). Inside-out subcellular vesicles from the d',o* mutant cannot oxidize ascorbate and TMPD, and cannot transport Na ° when suocinate is oxidized in the pre~nce of a protonophore. The same vesicles are found to transport Na ~ when NADH is oxidized as if the Na~-motive NADH-quinone oxidase were oi~rativ¢, On the other hand, a mutant lacking o oxidase (d~,o -) grows at low ZlRH* conditions as fast as the maternal E. coli strain containing both d and o oxidases. Corresponding vesicles oxidize ascorbute and TMPD as well as saccinate, the oxidations being coupled to the protonophore-stimulated Na* transport, Growth in the presence of a protonophore is found to induce a strong increase in the d c~idase level in the maternal d',o* E. colt strain, It is concluded that oxidase of the d.type, rather than of the o-type, operates as a Ha* pump in E, coli grown under conditions unfavorable for the H" cycle, Low tl/~w; Na*-motive terminal oxidase; d-and o-type oxidases; Escherichia colt