Eigenmannia virescens is a South American freshwater species with an electric organ discharge (EOD) of the wave type. This social fish's EOD is masked by the intense
noise from close-by conspecifics. In spite of this, Eigenmannia detects external signals at lowest thresholds for a vertebrate octavolateralis sensory system; furthermore, Eigenmannia discriminates between stimulus waveforms which is unknown for mechanical sensory modalities. It is suggested that an analysis of beat signals in three steps underlies these sensory feats: (1) stimulus filtering and intensity assessment of a
single harmonic of an external signal, in spite of only weak tuning of the high-frequency part of the electrosensory system. (2) Assessment of the frequency difference between a fish's own EOD and that of an external signal. It is shown that temporal cues of the
mixed (beat) signal are sufficient, and that beat amplitude cues are not required; nor do they seem to be used by the fish. Steps 1 and 2 completed, a fish may change its EOD frequency (e.g., by a jamming avoidance response) for optimizing (3) the assessment of stimulus waveform from the beat signal, such as external female and male EODs. Sensory models for the assessment of frequency difference and stimulus waveform are presented.