Phase separation of symmetric diblock copolymers end-grafted to a wall is studied using a Monte Carlo simulation. The nearest-neighbor repulsion between the two types ofmonomers is quenched from chi = 0 to chi = 1 for chains of length N = 100 at near-melt densities. We present evidence that the two monomer species vertically segregate to form a three-layer structure that uniform in the grafting plane. This is qualitatively different from a binary mixture of grafted homopolymers, which laterally segregates to form a structure with concentration modulated in the grafting plane at a scale of the side-to-side fluctuation distance.