4-pyridoxolactonase from Mesorhizobium loti MAFF303099 has been overexpressed in Escherichia coli. The recombinant enzyme was purified and was crystallized by the sitting-drop vapour-diffusion method using PEG 4000 and ammonium sulfate as precipitants. Crystals of the free enzyme (form I) and of the 5-pyridoxolactone-bound enzyme (form II) grew under these conditions. Crystals of form I diffracted to 2.0 A resolution and belonged to the monoclinic space group C2, with unit-cell parameters a = 77.93, b = 38.88, c = 81.60 A, beta = 117.33 degrees. Crystals of form II diffracted to 1.9 A resolution and belonged to the monoclinic space group C2, with unit-cell parameters a = 86.24, b = 39.35, c = 82.68 A, beta = 118.02 degrees. The calculated V(M) values suggested that the asymmetric unit contains one molecule in both crystal forms.