The EPR study of KMnfMg(1-f)F3 single crystals with Mg doping of 0.0, 4.8, 9.8, 16.6, and 31.5 at.% has been made between 52 and 300 °K. The Néel temperature TN is observed to decrease as the Mg doping is increased, except for the 31.5 at.% Mg-doped sample where longrange ordering was not observed down to 52 °K. The high-temperature EPR linewidth also decreases with Mg doping. Both these effects are reasonably understood by assuming that the number of nearest magnetic neighbors of Mn++ decreases with Mg doping. In all cases the EPR line narrows as the temperature is lowered towards TN.