The authors have investigated the initial growth of MnAs layers by step-by-step epitaxy on GaAs(110) and GaAs(001). On both surfaces, MnAs nanocrystals developed as the initial stage of MnAs layer formation. Surprisingly, an ultrahigh density (similar to 1x10(12) cm(-2)) of the nanocrystals with a height of similar to 5 nm and a size of similar to 20 nm appeared on GaAs(110). On different surface orientations, the density and the size of the nanocrystals vary. The behavior of the nanocrystallizations can be explained by symmetry at the surface. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3610963]