The initial stages of molecular beam epitaxial (MBE) growth of GaAs and AlGaAs are directly observed by scanning reflection electron microscopy. Under deficient As4 flux, it is shown that dark areas appear within several layers of growth, but disappear within several seconds of growth interruption, indicating that they are droplets of Ga and Al. The distance between the droplets is approximately 1 μm, which is identical in GaAs and AlGaAs growth. Thus, the diffusion length of Ga and Al on a group III‐rich surface at around 600 °C is estimated to be about 5000 Å, which is the largest among previous reports. The role of droplet disappearance in interface flattening in alternating supply MBE is discussed.