The flavoenzyme L-aspartate oxidase from Escherichia coli was crystallized using the hanging-drop vapour-diffusion technique with PEG 4000 as precipitant. The crystals belong to space group P3121 (or P3221) with unit-cell parameters a = b = 84.9, c = 159.9 A. A solvent content of 42% corresponds to a monomer (60 kDa) in the asymmetric unit. A complete 2.8 A resolution data set was collected using a rotating-anode X-ray generator.