A self-filtering unstable resonator with magnification |M| = 5 has been applied to a pulsed UV-preionized XeCl excimer laser. In agreement with numerical results it is experimentally shown that less than two cavity round trips are needed to establish a steady state lowest-order mode. A diffraction-limited laser beam with a brightness of 1.5 x 10(13) W cm(-2) sr(-1) has been obtained.