Densities of Mn ions in epitaxial layers of (Ga,Mn)As were determined by anomalous x-ray diffraction, i.e., by a measurement of the dependence of the intensity of weak diffraction 002 on the photon energy around the MnK absorption edge. From the measured data it was possible to determine the density of Mn ions in substitutional positions and the difference in the Mn densities in two possible interstitial positions in the GaAs lattice. The data confirm the previous finding that the density of Mn interstitials in centers of Ga tetrahedrons decrease during post-growth annealing.