A Trypanosoma brucei gene has been identified that encodes a protein predicted to be a component of the trypanosome homologue of mitochondrial NADH:ubiquinone oxidoreductase (complex I). High homology was found to a 20-kDa component of the iron-sulfur protein fraction of bovine mitochondrial NADH:ubiquinone oxidoreductase and the products of the ndhK locus of Paramecium tetraurelia mitochondria and the NQO6 locus of Paracoccus denitrificans. The homology extends to several other proteins predicted to function as part of electron transport systems, including the psbG/ndhK gene products of chloroplast and cyanobacterial genomes which are thought to be subunits of a NADH:plastoquinone oxidoreductase involved in chlororespiration. The T. brucei ndhK counterpart is nuclearly encoded. An extended amino terminus of the T. brucei ndhK with structural similarity to mitochondrial presequences indicates that its transfer into mitochondria is likely. Stumpy and slender bloodforms and procyclic forms all possess similar levels of ndhK transcripts despite previous reports of stage-regulated expression of complex I-like activity.