Nucleoside diphosphate kinase (NDPK) catalyzes the transfer of the gamma-phosphate from nucleoside triphosphates to nucleoside diphosphates. In addition to biochemical studies, a number of crystal structures of NDPK from various organisms, including both native proteins and complexes with nucleotides or nucleotide analogues, have been determined. Here, the crystal structure of Ynk1, an NDPK from the yeast Saccharomyces cerevisiae, has been solved at 3.1 A resolution. Structural analysis strongly supports the oligomerization state of this protein being hexameric rather than tetrameric.