Using near-field scanning optical microscopy (NSOM), we observe an inhomogeneous distribution of submicron-sized structural defects at the fusion boundary of polishedSrTiO3bicrystal substrates. Both NSOM and scanning force microscopy show that these substrate defects cause the grain boundary of aYBa2Cu3O7thin film grown on the bicrystal to wander up to a micron in the film. These structural defects are shown to correlate qualitatively with the electrical characteristics of grain-boundary Josephson junctions patterned on theYBa2Cu3O7film. ©1997 American Institute of Physics.