GaAs layers were grown on patterned (111)BBAR substrates having (122)ABAR sidewalls with various arsenic fluxes at a fixed temperature of 480-degrees-C by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMGa) and metal arsenic. Vertical growth rate on the top and bottom (111)BBAR surfaces decreased rapidly as the arsenic flux was increased. For arsenic fluxes of 2.0 X 10(-3) Pa and more, only lateral epitaxy on the (122)ABAR sidewall was achieved. Real-time scanning microprobe reflection high-energy electron diffraction (mu-RHEED) observations showed that the surface smoothness of the epitaxial layer was maintained throughout the growth time under the optimized condition.