l(+)‐Lactate dehydrogenase (LDH) is a key enzyme in anaerobic metabolism which converts pyruvate to lactate. LDH from the hyperthermophilic archaebacterium Methanococcus jannaschii has been overexpressed in Escherichia coli and crystallized in two crystal forms at 297 K using 2‐methyl‐2,4‐pentanediol as precipitant. Type I crystals grew rapidly and diffracted to at least 2.8 Å Bragg spacing upon exposure to Cu Kα X‐rays. X‐ray diffraction data to 2.9 Å have been collected from a native crystal. The type I crystal is tetragonal, belonging to the space group P42212, with unit‐cell parameters a = b = 99.74, c = 170.00 Å. The asymmetric unit contains two LDH subunits, with a corresponding crystal volume per protein mass (Vm) of 3.05 Å3 Da−1 and a solvent content of 59.7%. Type II crystals, which grew more slowly, diffracted to at least 1.8 Å Bragg spacing upon exposure to Cu Kα X‐rays. X‐ray diffraction data to 1.9 Å have been collected from a native crystal. The type II crystal is orthorhombic, belonging to the space group P21212, with unit‐cell parameters a = 47.65, b = 125.10, c = 58.08 Å. The asymmetric unit contains a single LDH subunit, with a corresponding crystal volume per protein mass (Vm) of 2.50 Å3 Da−1 and a solvent content of 50.8%. Therefore, the type II crystal is more suitable for high‐resolution structure determination than the type I crystal.