Three crystalline forms of 3-isopropylmalate dehydrogenase from the moderate facultative thermophile Bacillus coagulans were obtained by hanging-drop vapor-diffusion methods. One of them, which had crystallized under slightly milder conditions than the others, was suitable for X-ray analysis. Its asymmetric unit contains one dimeric molecule and the solvent content is higher than in other protein crystals. The crystal structure was solved in a preliminary manner by the molecular-replacement technique.