A dramatic suppression of the light absorption by exciton-polaritons is observed after resonant excitation of the polaritonic modes in a multiple quantum well Bragg structure. We demonstrate that this is due to the formation of a dominant superradiant polariton mode. The stability of the superradiant mode is demonstrated at high excitation powers and at temperatures of up to 180 K. (C) 1999 Elsevier Science B.V. All rights reserved.