Time-resolved three-pulse photon echoes of the exciton in layered semiconductor GaSe are investigated by using upconversion technique. The time-resolved echo signal depends on the separation time between the first and second pulses. The shape of the echo signal changes as the separation time increases. The time-resolved signal also depends on the waiting time between the second and third pulses. For a short waiting time. the temporal evolution of the echo signal shows an ideal photon-echo-like (Gaussian-like) shape. On the other hand, for longer waiting times, the temporal shape of the echo signal changes dramatically and has an extra peak at the arrival time of the third pulse. This behavior can be explained by spectral diffusion in the three-pulse photon echo process. (C) 2001 Elsevier Science B.V. All rights reserved.