L-Amino-acid oxidases (EC 1.4.3.2) catalyse the stereospecific oxidative deamination of an L-amino-acid substrate to an alpha-keto acid with the production of ammonia and hydrogen peroxide. In this study, the crystallization and preliminary X-ray analysis of a bacterial L-amino-acid oxidase from Rhodococcus opacus (RoLAAO) is described. RoLAAO is a dimeric protein consisting of two identical subunits of 489 amino acids with a calculated molecular weight of 54.2 kDa and a non-covalently bound FAD molecule. RoLAAO was crystallized by the vapour-diffusion method in two different space groups: P2(1)2(1)2(1) (unit-cell parameters a = 65.7, b = 109.7, c = 134.4 A) and C222(1) (unit-cell parameters a = 68.3, b = 88.4, c = 186.6 A). Both crystal forms diffracted X-rays to a resolution of at least 1.6 A.