Biodiversity of tropical forests varies at spatial and temporal scales, but at what scale and how the assemblages differ when we consider the same kind of forest and the same altitude has been poorly documented. We examined the variation in species richness and composition of three subfamilies of scarab beetles (Dynastinae, Rutelinae, and Melolonthinae) along a northwest-southeast (NW-SE) transect of premontane rain forests in Costa Rica. Sampling was carried out at six sampling sites comprising three mountain ranges. Two forest sites were selected in each mountain range at the same altitudinal strip and same life zone. We conducted eight field surveys from June 2007 to January 2008. UV light traps were used to catch the beetles. We studied beetle diversity at three spatiotemporal levels: mountain range, forest site, and monthly trap. Species composition and richness varied at the three levels studied, and this variation was not necessarily related with geographic distance. Forest site and monthly trap levels showed higher variation in similarity indices than the mountain range level did. Additive partitioning of diversity showed values of diversity higher than expected only at the spatial levels with the mountain ranges showing the highest contribution to gamma diversity. Our results show high spatiotemporal variation in scarab beetle assemblages across Costa Rican premontane rain forests and underline that the analysis of a variety of spatiotemporal scales within a single kind of forest is crucial to understand scarab beetle biodiversity patterns in forest ecosystems.