Summary: The compositions of more than eighty synthetic members of the apatite group with the general formula A10(XO4)6Z2 have been used as a basis for classifying the apatites into three groups. The ratios of the mean sizes of the ‘A’ ions to those of the ‘X’ ions for all these compositions lie between the limits of 1·89 and 4·43. A discontinuity occurs between ratios of 2·50 and 2·60 and probably another between 3·25 and 3·35, and these discontinuities provide a structural basis for the division into the three groups. The groups are named after well-known mineral species occurring in each group, i.e. the vanadinite-svabites with an A : X ratio less than 2·5, the apatite-mimetites between 2·60 and 3·25, and the pyromorphites with A : X greater than 3·25.