Recent simulations suggest that the presence of filler particles in a phase separating blend can induce the development of composition waves having the symmetry of the filler particles. We investigate these predictions through atomic force microscopy (AFM) measurements on ultrathin (L approximate to 100 nm) polystyrene and poly(vinyl methyl ether) blend films containing a low concentration of model filler particles (silica particles having a nominal diameter approximate to 100 nm). The filled blend films were spun-cast on acid-cleaned silica wafers, and phase separation was induced by a temperature jump into the two-phase region (T approximate to 145 degrees C) of the bulk polymer blend. By rinsing off the polymer film with solvent, we show that the silica particles are associated with the substrate so that the filler particles represent a quenched disorder perturbation of the film phase separation. The presence of the filler particles leads to the development of circular composition waves ("target patterns") about the filler particles during the intermediate stage of phase separation. These target patterns disintegrate as the background spinodal phase separation pattern becomes much larger than the filler particles. Our observations are consistent with idealized two-dimensional Cahn-Hilliard-Cook simulations of the phase separation of polymer blends having a small concentration of filler particles.