We studied the influence of habitat loss and fragmentation in species diversity, population dynamics, and habitat use in a small mammal community in xeric palo loco (Senecio praecox) thickets at Reserva Ecologica del Pedregal de San Angel, a completely isolated nature reserve in Mexico City, Distrito Federal, from 1989 through 1990. It is a heterogeneous region, dominated by an old lava flow, with many boulders and crevices. Small mammals were captured using Sherman live traps in two 0.81-ha grids; each grid contained 10 rows and 10 columns, separated by 10 m. Nine species have disappeared since 1957 as a result of the loss of particular habitat such as grasslands, habitat fragmentation, and other causes. In our study, we recorded nine species; five species were exclusively recorded in the continuous native scrub of the reserve. Therefore, they are good indicators of the sites in the reserve that maintain the less perturbed habitats. Interestingly, no introduced rodents (i.e., roof rat (Rattus rattus) and house mouse (Mus musculus)) were found in the continuous scrub of the reserve. Only six were caught in our grids (three were relatively common, represented by more than 10 individuals). Reproduction and maximum population densities of all species were very seasonal and peak at time of available high food resources, suggesting that food availability was the limiting factor.