We examined the effect of frugivorous bats and birds on the population dynamics of the giant columnar cactus Neobuxbaumia tetetzo in the Tehuacán Valley, Mexico. Because successful seedling establishment occurs only beneath the canopies of shrubs and trees, we hypothesized that seed dispersal is a key process in the maintenance of its populations. We determined the dispersal effectiveness of different frugivores, considering the quantity and quality components of seed dispersal. We also evaluated the potential effects of each frugivore species on the finite rate of increase of N. tetetzo populations by modifying the fecundity values of a Lefkovitch matrix model. The bat Leptonycteris curasoae had the highest effectiveness whereas the bird Carpodacus mexicanus had the lowest. The estimated finite rates of increase calculated to evaluate the effects of frugivores on the population dynamics of the cactus differ marginally from unity, except when the effect of the bat L. curasoae was analyzed. Our results suggest that the bat Leptonycteris curasoae could be considered the legitimate dispersal agent of N. tetetzo, dispersing seeds directly to safe sites, and thus representing a key species in the ecology of this columnar cactus.