A high precision star tracker has been developed as part of the attitude measurement and control system of the German Roentgen-Satellit ("ROSAT"). A Peltier cooled CCD array in the focal plane of an 82 mm refractive lens is used to measure the position of up to 3 stars within a field of view (FOV) of 5.9° x 4.4°. The position of a fourth reference star provided by the telescope of the satellite as an alignment control can be monitored simultaneously. The CCD readout data are reduced by a fast preprocessor; subsequently a microprocessor com-putes the exact star position by pixel interpolation using a centroiding algorithm. Optical performance tests on the engineering model have demonstrated the capability to track stars of my = +6.5 with an accuracy of 2 arcsec and a noise equivalent angle of less than 1 arcsec with an update period of 1 sec. In scan mode the stars can be tracked with an accuracy of less than 10 arcsec at a scan rate of 5 arcmin/sec. The environmental tests of the system were completed in October 1985.