The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell. Surprisingly, the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances. The exchange interaction between Mn ions oscillates as a function of the distance between them and obeys the Ruderman-Kittel-Kasuya-Yosida analytic formula. The estimated Curie temperature is in good agreement with recent experiments, and the estimated effective magnetic moment is about 1.7 µB/Mn, in excellent agreement with the experimental values, (1.4–1.9) µB/Mn.