Nicotinamide mononucleotide adenylyltransferase catalyses the final step in the synthesis of nicotinamide-adenine dinucleotide (NAD(+)) by transferring the adenylyl moiety of ATP to nicotinamide mononucleotide (NMN) with the release of pyrophosphate. The human enzyme was crystallized in the presence of NAD(+). Crystals grew in the orthorhombic space group C222(1), with unit-cell parameters a = 140.3, b = 235.5, c = 89.3 A, and diffract to a maximum resolution of 3.0 A. Packing considerations suggest a trimer or higher multimer to be present in the asymmetric unit of the crystal. Two archaeal homologues have been described to form hexamers.