The crystal structure of a copper amine oxidase fromArthrobacter globiformiswas determined at 1.08 Å resolution with the use of low-molecular-weight polyethylene glycol (LMW PEG; average molecular weight ∼200) as a cryoprotectant. The final crystallographicRfactor andR_freewere 13.0 and 15.0%, respectively. Several molecules of LMW PEG were found to occupy cavities in the protein interior, including the active site, which resulted in a marked reduction in the overallBfactor and consequently led to a subatomic resolution structure for a relatively large protein with a monomer molecular weight of ∼70 000. About 40% of the presumed H atoms were observed as clear electron densities in theF_o−F_cdifference map. Multiple minor conformers were also identified for many residues. Anisotropic displacement fluctuations were evaluated in the active site, which contains a post-translationally derived quinone cofactor and a Cu atom. Furthermore, diatomic molecules, most likely to be molecular oxygen, are bound to the protein, one of which is located in a region that had previously been proposed as an entry route for the dioxygen substrate from the central cavity of the dimer interface to the active site.