A Ge-GaAs superlattice with individual layer thicknesses of 10 nm (Ge layers) and 8 nm (GaAs layers) grown on a (100) oriented GaAs substrate at 400°C by molecular beam epitaxy was examined by transmission electron microscopy. Cross-sectional lattice images of the superlattice revealed no lattice irregularities in both GaAs and Ge layers and perfect lattice match at the Ge-GaAs interfaces. However, bright field and dark field images of the same superlattice indicated the presence of planar defects in the GaAs layers but not in the Ge layers. These planar defects, whose nature does not involve perturbations of the crystal lattice other than elastic strain fields, are most likely antiphase boundaries, which are generated during the growth of the GaAs layers by the coalescence of GaAs domains independently grown on the Ge surface. The generation of antiphase boundaries in GaAs layers might be responsible for the segregated Ge and GaAs columnar structure previously observed in Ge-GaAs superlattices with individual layer thicknesses of only a few monolayers.