Saccharomyces cerevisae ARO9 protein, an aromatic aminotransferase II, catalyzes the transamination step of the catabolism of aromatic amino acids, mainly tryptophan. ARO9 also belongs to a novel subfamily of enzymes within the aminotransferase subgroup I. Crystals of ARO9 protein have been grown using the hanging-drop vapour-diffusion method. The crystals belong to the orthorhombic space group P212121, with unit-cell parameters a = 75.6 Å, b = 117.5 Å, c = 134.9 Å. Diffraction data were collected to a resolution of 2.6 Å using a rotating-anode X-ray source. Analysis indicates the presence of two molecules in an asymmetric unit.