Dihydrodipicolinate reductase (DHDPR, DapB) is an enzyme that belongs to the L-lysine biosynthetic pathway. DHDPR reduces the alpha,beta-unsaturated cyclic imine 2,3-dihydrodipicolinic acid to yield the compound 2,3,4,5-tetrahydrodipicolinic acid in a pyridine nucleotide-dependent reaction. The substrate of this reaction is the unstable product of the preceding enzyme dihydrodipicolinate synthase (DHDPS, DapA). Here, the structure of apo-DHDPR from Mycobacterium tuberculosis is reported in two orthorhombic crystal forms, as well as the structure of DHDPR from M. tuberculosis in complex with NADH in a monoclinic crystal form. A comparison of the results with previously solved structures of this enzyme shows that DHDPR undergoes a major conformational change upon binding of its cofactor. This conformational change can be interpreted as one of the low-frequency normal modes of the structure.