We have studied the properties of MBE-grown MnAs layers on GaAs(111)B. In this report, we focus on the morphology of layers in the thickness range 1-20 ML, which we have probed by means of in situ electron diffraction (reflection high energy electron diffraction (RHEED) and LEED), and ex situ atomic force microscopy (AFM), and on the influence of the growth conditions (substrate temperature and growth rate). We found out that MnAs films, thinner than around 75 Angstrom, are discontinuous, with uncovered regions of the substrate, The MnAs islands are very flat, with roughness beyond the resolution of the AFM measurement. In layers thicker than 75 Angstrom, the areal density of uncovered GaAs substrate regions depends strongly on the MnAs growth temperature. Under particular conditions, continuous films of this thickness can be obtained.