[en] The Antarctic psychrophile Pseudoalteromonas haloplanktis produces a cold-active cellulase. To date, a three-dimensional structure of a psychrophilic cellulase has been lacking. Crystallographic studies of this cold-adapted enzyme have therefore been initiated in order to contribute to the understanding of the molecular basis of the cold adaptation and the high catalytic efficiency of the enzyme at low and moderate temperatures. The catalytic core domain of the psychrophilic cellulase CelG from P. haloplanktis has been expressed, purified and crystallized and a complete diffraction data set to 1.8 Angstrom has been collected. The space group was found to be P2(1)2(1)2(1), with unit-cell parameters a = 135.1, b = 78.4, c = 44.1 Angstrom. A molecular-replacement solution, using the structure of the mesophilic counterpart Cel5A from Erwinia chrysanthemi as a search model, has been found.