Low Ho3+ concentrations (∼0.01%) yield only the 1∣11 and 1∣12 (monomer) configurations of Ho3+–F– centres in both CaF2 and SrF2 crystals. The 1∣11 and 1∣12 configurations correspond to the charge compensating F– ion occupying the nearest neighbour and next nearest neighbour position respectively, relative to the Ho3+ ion. For CaF2:Ho3+ crystals, the two monomer centres occur with comparable densities while the 1∣11 centre is dominant in SrF2:Ho3+ crystals. At moderate Ho3+ concentrations (∼0.1%), Ho3+ dimer centres are dominant in both CaF2 and SrF2 and the population of monomer centres is significantly reduced. In CaF2:Ho3+, the 2∣21 and 2∣22 configuration dimer centres occur in comparable densities; these rhombic centres are formed by preferential aggregation of the 1∣11 and 1∣12 configuration centres, respectively. A third dimer centre, of the 2∣31 configuration, also appears in CaF2. Only the 2∣21 and 2∣31 configuration centres are present in similarly doped SrF2 crystals. The type and population of dimer centres that form in Ho3+–doped fluorite crystals depend on the monomer centre(s) favoured by the particular lattice. The crystalfield energy level structures of the 2∣21 and 2∣31 configuration centres are consistent with weaker crystal field effects, particularly in CaF2, compared to the parent 1∣11 centre; those of the 2∣22 and 1∣12 centres are comparable.