Crystals of recombinant obelin, the Ca(2+)-regulated photoprotein from the marine hydroid Obelia longissima, have been grown from sodium citrate solutions. Crystals grow as hexagonal light-yellow rods (0.1 x 0.1 x 1.0 mm) which diffract to beyond 1.8 A with synchrotron radiation of 1.0 A wavelength. The crystals have a primitive hexagonal lattice with unit-cell parameters a = 81.55, c = 86.95 A. The asymmetric unit contains two molecules. This represents the successful preparation of single crystals of a photoprotein obelin which have promising diffraction properties.