We describe a new frog species of the Mantidactylus boulengeri group (Amphibia: Anura: Mantellidae) in the subgenus Gephyromantis from south-eastern Madagascar. It is morphologically similar to M. eiselti and M. thelenae but differs in its advertisement calls with a distinctly shorter note duration, and in its bilobate (not single) subgular vocal sac. Like M. eiselti and M. thelenae the new species Mantidactylus enki is predominantly diurnal and calling males do not aggregate close to water bodies, indicating direct development as it has been demonstrated previously in M. eiselti. The advertisement calls of all three species are described in detail. A comparison with a further pair of cryptic species with bilobate vs. single subgular vocal sac (Mantidactylus tschenki — M. cornutus) did not reveal any consistent pattern of call variation correlated with vocal sac structure. Therefore, sexual selection may also be considered to explain differences in the colour and external structure of vocal sacs among closely related species.