This study shows that the plant mitochondrial nucleoside diphosphate kinase (mNDPK) localizes to both the intermembrane space and to the mitochondrial inner membrane. We show that mNDPK is very firmly attached to the membrane. Co-immunoprecipitation experiments identified the adenine nucleotide translocator as an interaction partner. This is the first report showing a direct association between these two proteins, although previous studies have shown metabolic cooperation between them. Possible consequences for mitochondrial energy metabolism are discussed.