We report on the evolution of the growth front during molecular-beam epitaxy on GaAs (3 1 1)A substrates stripe patterned along the [ 01(1) over bar] direction as a function of the mesa height. During growth (1 0 0) and (2 1 1)A facets are formed and expand at the corners near the two opposite lying (1 (1) over bar (1) over bar )A and (1 1 1)A mesa sidewalls, respectively. After covering the (1 (1) over bar (1) over bar )A facet completely, the (1 0 0) facet shrinks away, leaving behind a convex surface profile. For shallow mesa height of several ten nanometers, the growth develops a fast-growing microscopic slope tilted several degrees to (3 1 1)A, which is identified as the basis for quantum wire formation. O 2000 Elsevier Science B.V. All rights reserved.