(Uploaded by Plazi for the Bat Literature Project) Neotropical bats exhibit great diversity in terms of resource use, so their species richness can vary both spatially and temporally over fine and coarse scales. In this paper we analyze the relative contribution of the alpha (F) and beta (G) components of bat species richness at different spatial and temporal scales in deciduous and semideciduous tropical forests in Yucatan, Mexico. We used different levels of the spatial (understory and subcanopy height, site, and vegetation type) and temporal (sampling period within a night, sampling night, and season) dimensions. For the spatial dimension, we found that the finer level (F) accounted for more than 50% of total richness and was higher (72% of total richness) within the semideciduous tropical forest. Although lower than for F, the percentage contribution of G among sites and G between vegetation types was higher than expected by chance. For the temporal dimension, F contribution was lower than 25% of total richness in all cases, while G at different time scales had a great influence on bat species richness. Our results indicate that the species composition of bat assemblages is dynamic in space and time. Thus, a proper assessment of diversity requires the inclusion of a variety of spatial and temporal scales in sampling designs.