Cystathionine β-synthase (CBS) is a pyridoxal-5′-phosphate-dependent enzyme that catalyzes the first step of the transsulfuration pathway, namely the condensation of serine with homocysteine to form cystathionine. Mutations in the CBS gene are the single most common cause of hereditary homocystinuria, a multisystemic disease affecting to various extents the vasculature, connective tissues and central nervous system. At present, the crystal structure of CBS fromDrosophila melanogasteris the only available structure of the full-length enzyme. Here we describe a cloning, overexpression, purification and preliminary crystallographic analysis of a full-length CBS fromApis mellifera(AmCBS) which maintains 51 and 46% sequence identity with itsDrosophilaand human homologs, respectively. TheAmCBS yielded crystals belonging to space groupP2₁2₁2₁, with unit-cell parametersa= 85.90,b= 95.87,c= 180.33 Å. Diffraction data were collected to a resolution of 3.0 Å. The crystal structure contained two molecules in the asymmetric unit which presumably correspond to the dimeric species observed in solution.