We conducted a demographic examination of Pterocereus gaumeri, a rare and endemic columnar cactus of the Yucatan peninsula, Mexico. Growth, survival, and fecundity of individuals were recorded in two populations from 1997 to 1999. Size-based population projection matrices were constructed to document the population dynamics. Seed germination and seedling establishment experiments were conducted in the field to estimate the rate of seed and seedling survival. Our results show that P. gaumeri did not reproduce by asexual means or vegetative growth and thus relies wholly on sexual reproduction for population growth. Low reproductive success, seed predation, and low seedling survival appear to be important factors that contribute to the rarity of this species. The population growth rate (λ) varied between 0.9545 and 1.0316. According to the matrix elasticity values, population dynamics of this species depend mainly on adult survival, with low values in fecundity and survival for the lowest size categories. Our results did not indicate that the population size of P. gaumeri was decreasing; however, increasing pressure on land use transformation is one of the most important factors that may threaten the persistence of this species.