The present article addresses the deterioration of 12 forest geosystems located in a mountain range to the west of Mexico City. On the basis of identification of plant species that are indicators of environmental deterioration, as well as application of a Deterioration Index that considers these species in relation to the richness, vertical structure, and total cover of the climax forest facies—representing the most developed and stable evolutionary stage of each geosystem—forest geosystems are classified according to the present stage of deterioration that affects the best preserved forests in each case.