Abstract: The 46 kDa xylanase from the Antarctic microorganism Pseudoalteromonas haloplanktis is an enzyme that efficiently catalyzes reactions at low temperatures. Here, the crystallization of both the native protein and the SeMet-substituted enzyme and data collection from both crystals using synchrotron radiation are described. The native data showed that the crystals diffract to 1.3 A resolution and belong to space group P212121, with unit-cell parameters a = 50.87, b = 90.51, c = 97.23 A. SAD data collected at the peak of the selenium absorption edge proved to be sufficient to determine the heavy-atom configuration and to obtain electron density of good quality.