We report density-functional calculations of the ferromagnetic (FM) stabilization energy δ=EFM-EAFM for differently oriented Mn pairs in Ⅲ-V's (GaN, GaP, GaAs) and chalcopyrite (CuGaS2, CuGaSe2, CuGaTe2) semiconductors. Ferromagnetism is found to be the universal ground state (δ＜0) in all cases. The order of FM stability in Ⅲ–V’s is GaN＞GaP＞GaAs, whereas in chalcopyrites it is CuGaS2＞CuGaSe2＞CuGaTe2. Considering both groups, the order is GaN →GaP→GaAs→CuGaS2→CuGaSe2→GaSb≈CuGaTe2. The stronger FM stabilization in Ⅲ-V's is attributed to the stronger covalent coupling between the Mn 3d and the anion p orbitals. In contrast to expectations based on Ruderman-Kittel-(Kasuya)-Yosida, (i)all Mn-Mn pair separations show FM, with no FM to antiferromagnetic oscillations and, (ii) FM is orientationally dependent, with <110> Mn-Mn pairs being the most FM.