Hexose oxidase (EC ) fromHansenula polymorphawas found to exhibit a dual covalent association of FAD with His79 via an 8a-histidyl linkage as well as a covalent association between Cys138 and C-6 of the isoalloxazine moiety of FAD. Spectral properties of the wild-type enzyme exhibited maxima at 364 nm and 437 nm as well as a distinct shoulder at 445 nm.