Beta(2)-microglobulin (beta(2)m) is an essential subunit of MHC I molecules; it stabilizes the structure of MHC I and plays a pivotal role in coreceptor recognition. To date, structures of beta(2)m have been solved for three different mammals: human, mouse and cattle. In order to illuminate the molecular evolutionary origin of beta2m, an understanding of its structure in lower vertebrates becomes important. Here, grass carp (Ctenopharyngodon idellus) beta(2)m (Ctid-beta(2)m) was expressed, purified and crystallized. Diffraction data were collected to a resolution of 2.5 A. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 38.72, b = 40.65, c = 71.12 A. The Matthews coefficient and the solvent content were calculated to be 2.56 A Da(-1) and 52.07%, respectively, for one molecule per asymmetric unit. The structure has been solved by molecular replacement using monomeric human beta(2)m as a model.