We tested for a relationship between number of bat species and surface area of 20 caves in central Mexico and investigated the role of the habitat diversity model as an explanation for this relationship. There was a significant positive correlation between the logarithm of species richness and the logarithm of cave surface area, evidence of a species–area relationship. Our data suggest that roost site diversity, as indicated by spatial variation in relative humidity and presence of avons (conical depressions in cave ceilings) is a cause of the species–area relationship.