Abstract
Studies of pulse‐type gymnotiform electric fishes have suggested that electric organ discharge waveforms (EODw) allow individuals to discriminate between conspecific and allospecific signals, but few have approached this experimentally. Here we implement a phase‐locked playback technique for a syntopic species pair, Brachyhypopomus gauderio and Gymnotus omarorum . Both species respond to changes in stimulus waveform with a transitory reduction in the interpulse interval of their self‐generated discharge, providing strong evidence of discrimination. We also document sustained rate changes in response to different EODws, which may suggest recognition of natural waveforms.