The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band can explain ferromagnetism in GaAs(Mn) in both degenerate and nondegenerate samples. Forma-tion of ferromagnetic clusters and the percolation picture of phase transition describes well all available experimental data and allows us to predict the Mn-composition dependence of transition temperature in wurtzite (Ga, In, Al)N epitaxial layers.