The nucleation and growth of GaAs nanowires fabricated by molecular beam epitaxy (MBE) following the Au-assisted vapor-liquid-solid mechanism were compared on GaAs(1 1 1)B and on Si(1 1 1) substrates. On both substrates, reflection high-energy electron diffraction (RHEED) patterns and scanning electron microscopy (SEM) images of several samples belonging to a growth time series were analyzed. During the nucleation stage, growth on Si(1 1 1) is dominated by horizontally growing traces and coalescing islands, while growth on GaAs( 1 1 1)B proceeds instantly in the vertical direction. After this nucleation stage. the Si substrate is covered by a closed, rough GaAs layer, and nanowires of similar shape grow on both substrates with similar axial and radial growth rates. However, the diameter of the nanowires on Si(1 1 1) is different than that on GaAs(1 1 1)B, because the size of the Au droplets, which result from the annealing of a thin Au layer, is different on the two types of substrates. (C) 2010 Elsevier B.V. All rights reserved.