The profound effect of magnetic dilution in Zn1−xMnxGa2Se4 with 0<x<0. 5 is demonstrated by the analysis of neutron powder diffraction and magnetic susceptibility data. All the members of the series show a defective stannite crystal structure and a random distribution of the Mn ions. The magnetic archetype system of Zn1−xMnxGa2Se4 with 0<x<0. 5 is not found to be MnGa2Se4 (the magnetic end member of the series with I4¯ symmetry) but, instead, a hypothetical compound with I4¯2m symmetry. The distance between the Mn ions along the various magnetic superexchange pathways is not seriously affected by the degree of dilution. However, the global magnetic interaction between manganese ions is found to be significantly higher than that expected for the classical magnetic dilution of the parent MnGa2Se4. A generally applicable method for the calculation of the correct Weiss temperature θ in diluted magnetic semiconductors is provided.