Weakly electric fish are good model animals to study the evolution of interspecific and sexual differences in communication signals. This is because the neural circuits producing these signals are simple and conserved among related species while the signals are highly species-specific, sexually-dimorphic, and under hormonal control. Here we focus on two related species of weakly electric gymnotiform fish that emit a wave-type discharge. These species differ in the direction of the sexual dimorphism of their electric organ discharge (EOD) frequencies and their propensity to produce aggressive communication signals called 'chirps'. Brown ghost (Apteronotus leptorhynchus) males produce high frequency EODs while females produce low frequency EODs. When presented with an EOD mimic, males chirp frequently, while females seldom chirp. By contrast, black ghost (A. albifrons) males discharge at lower EOD frequencies than females, and there is no sex difference in chirping in this species. Accordingly, non-aromatizable androgens raise EOD frequency in brown ghosts, but lower it in black ghosts. Androgens induce chirping in female brown ghosts, but do not increase the propensity to chirp in female black ghosts. Thus, the difference in sexually-dimorphic communication signals between these two species can be explained by differences in their responses to sex steroids. Future studies will elucidate how the neural circuits generating these signals are differentially sensitive to steroids in these species.