A new trigonal crystal form of photolyase from the cyanobacterium Anacystis nidulans has been obtained by the addition of ethylenediaminetetraacetate to the crystallization condition previously used for the tetragonal crystals. In contrast with the tetragonal form that is grown as assemblies of small single crystals, isolated large single crystals were obtained for the present trigonal form, thus providing a regular supply of crystals for X-ray diffraction work. The new form of crystal belongs to the space group P3(1)21 or P3(2)21 with unit cell dimensions of a = b = 146 Angstrom and c = 134 Angstrom. Assuming that the asymmetric unit contains two or three molecules, the V-m value is calculated as 3.9 or 2.6 Angstrom(3)/Da, respectively. The present crystals diffract X-ray beyond 3.0 Angstrom resolution with synchrotron radiation and were stable toward X-ray exposure. (C) 1995 Academic Press, Inc.