In the Ajusco volcano, in Central Mexico, prescribed burnings of low and high intensity were applied in March and May 2002, along with one unburned control for March and another for May, considering conditions of open stands and closed stands, with the objective of evaluating, the vegetation response of the Pinus hartwegii understory. Samplings were conducted every three months during one year. Number of herb and shrub species, density, covering, and frequency were recorded, and the Simpson diversity index was calculated. According to a principal component analysis, the March prescribed burnings of low and high intensity in open stands separate from the other treatments. The t test showed that the March low intensity burns in open stands yielded a higher Spring-diversity than the unburned control. For the shrub species diversity, the March and May high intensity burns in open stands formed a separated group. A c2 test revealed the species that appear preferably in burned areas. The results indicate that the use of prescribed burnings no later than March favor the richness and diversity of species of the understory.