We studied the community of arthropods inhabiting the ecological preserve El Pedregal de San Angel, in Mexico City. We estimated morphospecies richness, abundance, and diversity patterns in two contrasting habitats, open and closed, during four seasons of the year. Because closed-habitat sites have greater net primary productivity than open habitats, we predicted arthropod diversity would be greater in closed than in open habitats. Morphospecies richness of Acari and Arachnida was similar in both types of habitats throughout the year, whereas richness of some orders of Insecta such as Coleoptera, Diptera, Hemiptera, and Hymenoptera was greatest in open habitats. Mites were the most abundant arthropods and were always more abundant in open than closed habitats. Diversity of arthropods was greater in closed habitats during fall and spring, suggesting that habitat characteristics related to primary productivity might affect this community. We suggest that studies of patterns of arthropod diversity and factors associated with the maintenance of such diversity should be considered in the preservation of these organisms and of their natural habitats.