Dutch elm disease fungus Ophiostoma novo-ulmi contains a hydrolase activity which catalyses the resolution of racemic ethyl naproxen to the corresponding acid. The recombinant enzyme has been crystallized by the vapour-diffusion method in two crystal forms. The crystals of the first form belong to space group P2(1)2(1)2, with unit-cell parameters a = 115.9, b = 174.4, c = 62.1 A. The enzyme also crystallizes in space group P2(1)2(1)2, with unit-cell parameters a = 72.9, b = 212.7, c = 61.7 A. Synchrotron data have been collected for both crystal forms to 2.6 and 2.3 A, respectively. A molecular-replacement solution has been found using a remote starting model of a bacterial esterase (23% sequence identity) for both crystal forms. Multicrystal averaging has resulted in interpretable electron-density maps.