Black-bellied wrens (Thryothorus fasciatoventris) use loud songs to communicate sex over long distances. We compared male and female songs recorded from a central Panamanian population of black-bellied wrens. All nine measured features differed significantly between the sexes. Males sang lower fundamental frequencies than females, but this difference cannot be explained by simple body size-frequency scaling. A discriminant function analysis correctly discriminated the singer's sex for all songs in the analysis. When viewed as sonograms, the terminal syllables of male and female songs exhibited opposite structure – all male songs ended in V-shaped syllables, and all female songs ended in arc-shaped syllables. The degree and character of dimorphism lead us to describe song structure in this population as ‘sexually antithetical’. Variation in song dimorphism throughout this species' range provides an excellent opportunity to test the hypothesis that signal degradation during transmission selects for divergent signal structure.