Patterns of interspecific and intraspecific variation in the three endemic species of Malagasy Triaenops bats were investigated using morphology and bioacoustics. Adult bats were captured at different localities across the island, measured, and their echolocation calls recorded. On average, male T. auritus and T. furculus have shorter forearms (47.0 and 44.0 mm, respectively) and emit higher frequency calls (107.8 and 113.1 kHz, respectively) than females (47.5 and 45.7 mm and 95.6 and 98.2 kHz, respectively), representing a form of reversed sexual dimorphism (females larger than males). However, T. menamena shows typical patterns of sexual size dimorphism with males having a longer forearm (51.7 mm) and lower frequency echolocation calls (82.3 kHz) than females (49.0 mm and 93.5 kHz, respectively). When segregated by sex, there was a strong allometric relationship between forearm length, used as a measure of body size, and the resting frequency in these three species, as well as two African hipposiderids (T. afer and Cloeotis percivali). Triaenops auritus males and both sexes of T. furculus deviated from the relationship between these two variables. Hypotheses are explored to explain the drivers of these sexual dimorphism patterns. On the basis of the allometric relationship, the strong correlation is in parallel to other groups of bats and is probably associated with ecological constraints. Recent phylogenetic analyses showed a separation of Afro-Malagasy Triaenops into two sister clades: T. auritus/T. furculus (suggested to be placed in a new genus, Paratriaenops) and T. menamena/T. afer. The patterns of sexual dimorphism in these taxa are congruent with clade membership. Further studies are needed to understand strategies used by these taxa when in sympatry to share habitat and ecological niches.