MnAs epilayers were grown by molecular beam epitaxy on GaAS(1 1 1)B substrates. The morphology of epilayers has been studied by coupling several in situ techniques. Two distinct growth regimes were distinguished as a function of the substrate temperature. For the growth at 320 degreesC. the system shows an intriguing mechanism of relaxation that produces MnAs isolated islands (the so-called "blocks") with constant height. The explanation for this mechanism associates the large mobility of atoms at this temperature with the strain due to the important misfit. At lower temperature (200 degreesC) the surface mobility is greatly reduced which results in a more homogeneous film. (C) 2002 Elsevier Science B.V. All rights reserved.