The crystal structure of the copper-containing enzyme, galactose oxidase, has been solved by multiple isomorphous replacement and refined to a resolution of 1.7 A. The X-ray structure reveals a unique polypeptide fold. The protein can be divided into three domains, all of which consist almost entirely of beta-strands. The structure of the second domain is particularly striking, 28 beta-strands arranged in a pseudo 7-fold symmetry. The copper site is on the surface of the protein and extremely rich in aromatic side-chains. The copper ion has two histidines, two tyrosines, and one external ligand in distorted square pyramidal coordination. The presence of pyrroloquinoline quinone as a covalently bound cofactor in GOase has been excluded. Instead, an unexpected covalent linkage between Tyr272 and Cys228 has been observed, whose functional role may relate to the presence of a tyrosine free radical at Tyr272. The tyrosine free radical could be stabilized by delocalization to Cys228 and stacking interactions with Trp290. A structural model for substrate binding is proposed that offers an explanation for the substrate specificity of the enzyme and many of the spectroscopic and enzymological data. Although the model lacks direct confirmation at present, it should provide a stimulus for further spectroscopic and crystallographic studies.