Five new specimens of the poorly known Sanborn's bonneted bat, Eumops hansae (Chiroptera: Molossidae), are reported for Costa Rica's central and northern Pacific lowlands based on a subadult male collected in 1990 and four adult females collected in 2003. We also report the second known specimen of Eumops underwoodi from Costa Rica and additional specimens of Cynomops mexicanus, Eumops glaucinus, Molossus molossus, and Molossus pretiosus. Most of the females captured in August and April were either lactating or pregnant, suggesting that parturition in these molossids occurs in the late dry season and the early to middle rainy season, periods when insects are especially abundant in this dry forest. Characters used previously to distinguish between the similar-sized E. hansae and E. nanus are evaluated, and external and cranial measurements for the specimens of E. hansae are provided. The best single character for distinguishing the two species is size and shape of the upper incisors. In E. hansae, the upper incisors are thin and recurved, whereas they are thick, straight, and slightly procumbent in E. nanus. The six sympatric species of free-tailed bat found in the gallery forest along the Río Enmedio vary in size, jaw thickness, and wing shape suggesting coexistence through resource partitioning in this molossid bat assemblage.