A beam vertex detector system based on 200 mu m square scintillating fibers has been built and utilized in an experiment with 6,8, and 10.8 GeV/u Au-197 beams with intensities of up to 3 x 10(5) particles per 1.6s spill at the AGS at Brookhaven National Laboratory. The position of each beam particle traversing the target which subsequently caused a trigger was determined with a precision of sigma similar or equal to 150 mu m, independent of beam intensity. The detection efficiency exceeded 99% per detector layer. Information on radiation damage with relativistic heavy ions was obtained. (C) 1998 Elsevier Science B.V. All rights reserved.