Summary: The electron spin resonance spectrum of a sample of natural, orange-coloured fluorite from Broken Hill, Australia, has been observed and shown to arise from the presence of manganese ions. From an analysis of the structure of this spectrum it has been possible to estimate the various contributions to the ionic energy level splitting and to show that these are in good agreement with comparable results for synthetic material. It is evident that the manganese ions have entered the lattice substitutionally at Ca²⁺ sites and the nature of this site is discussed.