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We infer the role of vocalization in male territorial behaviour in an inconspicuous frog species (Gephyromantis thelenae). We test for differences in the vocal response of males to playback of conspecific calls versus calls of the syntopic sister species (G. eiselti). In addition, we describe a novel “aggressive” call type, produced to challenge territorial intruders. We found G. thelenae males to significantly increase their vocal activity towards conspecific playback, sometimes accompanied by positive phonotactic behaviour. A few males reduced their vocal response relative to control recordings, and also tended to show negative phonotaxis. These differences in territorial behaviour may be related to differences in call parameters affected by male quality. This indicates that male G. thelenae probably do not perceive calls of G. eiselti as conspecific competitors. Such acoustic niche partitioning might be the result of divergent sexual selection on advertisement calls that could have promoted speciation in these two closely related, syntopic species, but alternate explanations cannot be ruled out at this point until preferences of females are assessed and more data on population genetics are available.