Bat diversity and activity have been documented to change with elevation and to be correlated with temperature variation, especially in temperate regions. Acoustic elevational studies of insectivorous bat fauna in tropical mountains are scarce. Here, we investigated diversity, general and species-specific activity patterns of aerial insectivorous bats along an elevational gradient in Costa Rica and tested the hypothesis that on tropical wet mountains species richness of insectivorous bats follows a decreasing pattern with elevation. We expected a peak of species diversity and activity at lower elevations and higher temperatures and no major differences between months. With simultaneous acoustic monitoring on five elevations, we recorded 11 984 bat passes during approximately 550 recording hours and identified 15 species, two genera and one sonotype at the family level. We found the highest diversity and activity of bats at mid-elevations, besides elevational, temporal and species-specific differences that were partially explained by temperature. The fact that in our transect humidity does not change with elevation may explain observed differences from the expected patterns. Climate change in Monteverde could be one of the factors resulting in elevational movements of bats to mid-elevations, probably affecting the availability of food resources, and thus the foraging activity of aerial insectivorous bats.