Ga1-xMnxAs layers with Mn composition of up to 6.2% are investigated by cross-sectional scanning tunneling microscopy and spectroscopy. We identify in the tunneling spectra contributions from MnGa- acceptor states, compensating AsGa2+ donor states, and additional compensating donor states, which we suggest to be Mni2+ interstitials. On basis of the observed Fermi level shift and a charge carrier compensation analysis, we deduce the concentration of Mni2+ interstitials. Furthermore, scanning tunneling microscopy images suggest an inhomogeneous distribution of Mn dopant atoms. © 2003 American Institute of Physics.