Epitaxial Si layers grown on (111) Si substrates by vapor decomposition methods often exhibit a triangleand/or line type of surface defect. An investigation of such defects by optical microscopy and particularlytransmission electron microscopy has shown the defects to be stacking faults. The stacking faults usuallycommence at certain points lying in or close to the substrate/layer interface, and develop in a regular manneralong one or more of the three inclined {111} planes. As growth proceeds the stacking faults rapidly increasein area, spread laterally and eventually interact with one another. A simple growth mechanism is proposedbased on the assumption that nucleation centers occasionally go down in incorrect sequence, i.e., small areasof stacking fault form on the (111) plane parallel to the substrate/layer interface. Many of the experimentalobservations can be explained using this model.