A bound on the probability of error is obtained for anM-ary direct-detection optical communications system consisting of an amplitude-stabilized source, a lognormal atmospheric channel, and a photocounting detector array. Equal-energy, equiprobable, orthogonal signaling, and fiat independent fading at all detectors is assumed. The result reduces to that obtained previously in the absence of fading. A comparison is made with the analogous solution for the heterodyne array receiver.