Fission-track dating and K-Ar dating indicate that the age of apatite from Cerro de Mercado, Mexico, is 30 m.y., in contradiction to previous corrected fission track ages of 40 and 57 m.y. by other works Annealing data for the ''plateau method'' correction of fission-track ages for the Cerro de Mercado apatite lead to corrections by a factor of sixty or more, which give geologically unreasonable ages. In addition, published data concerning the length of fission tracks and the annealing of minerals imply that the basic assumptions used in an alternative procedure, the length reduction-correction method, are also invalvid for many crystal types and must be approached with caution unless individually justified for a particular mineral.