The characteristics of an Al‐Ga top layer on AlGaAs and GaAs surfaces during alternate supply molecular beam epitaxy (MBE) growth are studied byinsituobservation using a MBE scanning electron microscope (SEM) hybrid system. It is found that an Al‐Ga alloy top layer has a pseudo‐self‐limiting nature. It is also found that the migration distance of Al‐Ga atoms on an Al‐Ga alloy top layer is as large as 10 &mgr;m. By utilizing these characteristics, a &mgr;m‐scale Al‐Ga monolayer lateral growth process is realized, and the lateral growth rate, about 1 &mgr;m/s, is observed. Comparison of this growth with migration‐enhanced epitaxy is discussed.