Crystals of the glycogen synthase (GS) from Agrobacterium tumefaciens have been grown that diffract to 2.6 A resolution. The enzyme, which is homologous to the starch synthases of plants, catalyzes the last reaction step in the biosynthesis of glycogen. It is a alpha-retaining glucosyltransferase that uses ADP-glucose to incorporate additional glucose monomers onto the growing glycogen polymer. Its homology with mammalian GSs is marginal, but several regions shown to be important in catalysis are strictly conserved. Knowledge of the crystal structure of GS will be a major advance in the understanding of glycogen/starch metabolism and its regulation. A rational approach in enzyme engineering can subsequently be envisaged. The multiwavelength anomalous diffraction approach will be used to solve the phase problem.