We describe the structure and expression of a wheat mitochondrial gene, which codes for a subunit of mitochondrial NADH dehydrogenase. The deduced protein sequence has 70% similarity to the 30-kDa subunit of bovine mitochondrial complex I and 65% similarity to the 31-kDa subunit of Neurospora crassa complex I, components of the iron-sulfur-protein fraction, both nuclear-encoded proteins. We named this wheat mitochondrial gene as nad9. The wheat nad9 gene is transcribed in a single mRNA of 0.9 kb that is edited (C-to-U conversions) in 14 positions. Transcript mapping revealed that the first ATG codon is just 20 nucleotides downstream of the mRNA 5' end and that the 3' end is just 23 nucleotides downstream of the nad9 stop codon. The expression of the nad9 gene in plant mitochondria was studied. Polyclonal antibodies prepared against a wheat NAD9 fusion protein specifically recognise the 30-kDa subunit of bovine mitochondrial complex I and a 27.5-kDa protein in the membrane fractions of wheat, maize and common bean mitochondria, whereas the same serum recognizes a 30-kDa protein in the mitochondria of pea, chickpea and lentil.