The structure of apo malate dehydrogenase from Escherichia coli has been determined to 1.45 A resolution. The crystals belonged to space group C2, with unit-cell parameters a = 146.0, b = 52.0, c = 168.9 A, beta = 102.2 degrees. The structure was determined with the molecular-replacement pipeline program BALBES and was refined to a final R factor of 18.6% (R(free) = 21.4%). The final model has two dimers in the asymmetric unit. In each dimer one monomer contains the active-site loop in the open conformation, whereas in the opposing monomer the active-site loop is disordered.