We prepared epitaxial Sr2FeMoO6 thin films on SrTiO3 (100) substrates by pulsed laser deposition. With x-ray diffraction we investigated the crystallographic properties. The films have a tetragonal unit cell and are (00l) oriented, i.e. perpendicular to the substrate plane. Already at a deposition temperature TD of 320 degreesC a high degree of epitaxy was achieved. Depending on preparation parameters a large variation of the c-axis length was found. At a high (low) deposition temperature of 930 degreesC (320 degreesC) we identified a rock-salt (random) Fe,Mo cation arrangement. The differences in crystal structure go along with respective differences in magnetisation and transport properties. Both semiconducting and metallic behaviour was found. Only at high deposition temperatures of 930 degreesC films are fully metallic with a negative magnetoresistance up to room temperature and a saturation magnetisation of 4 mu (B) as expected for an ideal ferrimagnetic arrangement.