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Simple Summary: How the diversity of species changes from place to place is well known for some groups, such as vertebrates, considerably aiding conservation planning. However, it is often poorly known for the very diverse groups that make up most species on Earth, such as many invertebrates. This may hinder their effective conservation. We surveyed a group of "Darwin wasps", a very diverse and important component of invertebrate fauna, up a mountain in the Brazilian Atlantic Rainforest, a global hotspot for biodiversity. The wasps reproduce by parasitizing other insects and spiders. We found a large number of species of these wasps, suggesting that biodiversity may be high for this group in tropical regions, despite some previous studies suggesting the opposite. We found that low- and mid-altitude locations were especially diverse for this group, but not high-altitude locations. Furthermore, different species were found at different altitudes up the mountain. These findings suggest that tropical forests may harbor high concentrations of Darwin wasps; consequently, deforestation risks losing much of that biodiversity. Conserving low- to mid-altitude forests may be the most effective way to conserve the diversity of these wasps, although protecting a wide range of altitudes is necessary to conserve all species. Understanding how biodiversity varies from place to place is a fundamental goal of ecology and an important tool for halting biodiversity loss. Parasitic wasps (Hymenoptera) are a diverse and functionally important animal group, but spatial variation in their diversity is poorly understood. We survey a community of parasitic wasps (Ichneumonidae: Pimplinae) using Malaise traps up a mountain in the Brazilian Atlantic Rainforest, and relate the catch to biotic and abiotic habitat characteristics. We find high species richness compared with previous similar studies, with abundance, richness, and diversity peaking at low to intermediate elevation. There is a marked change in community composition with elevation. Habitat factors strongly correlated with elevation also strongly predict changes in the pimpline community, including temperature as well as the density of bamboo, lianas, epiphytes, small trees, and herbs. These results identify several possible surrogates of pimpline communities in tropical forests, which could be used as a tool in conservation. They also contribute to the growing evidence for a typical latitudinal gradient in ichneumonid species richness, and suggest that low to medium elevations in tropical regions will sometimes conserve the greatest number of species locally, but to conserve maximal biodiversity, a wider range of elevations should also be targeted. [ABSTRACT FROM AUTHOR]