A two-kilobase DNA fragment from Buchnera aphidicola, the endosymbiont of aphids, was cloned and sequenced. One open reading frame was detected, coding for a putative protein of 600 amino acids. The N-terminal portion of this protein corresponded to NuoC, while the C-terminal portion corresponded to NuoD. These proteins are constituents of the membrane-associated NADH dehydrogenase. Our results suggest that these two proteins are fused in Buchnera aphidicola, a result consistent with their previously postulated spatial association.