We explored coastal streams, rivers, and swamps in the Guianas of South America and found eleven species of gymnotiform fishes with pulse discharges. Each species has a characteristic electric organ discharge (EOD) waveform of 0.5-5 ms duration; at least two species appear to have a natural sex difference in their EODs which is apparent when comparing large adult males and females. Three sensory coding mechanisms are proposed to explain how electric fish might be able to determine species and sex identity from such short electrical pulses. Spectral Coding: electroreceptors tuned to different frequencies encode the spectrum of the EOD as a cross-fiber stimulation pattern. Temporal Coding: EODs are encoded as a volley of nerve spikes patterned in the time domain. Scan Sampling: a receiver detects a signaler's EOD as an amplitude modulation or 'beat' set up by the combination of its own discharge with the signaler's. The receiver uses the modulation envelope to assess the signaler's EOD waveform. To distinguish between these three coding mechanisms, we tested the ability of one pulse gymnotiform, Hypopomus beebei, to discriminate one electric waveform from another by comparing the acceleration of the discharge rate to different stimuli. Stimuli are presented under two conditions: when the stimulus pulse train is free-running compared to the fish's pulse train, and when the stimulus train is phase-locked to the fish's discharge pulse train. Under the former condition scan sampling may be used; under the latter it will be impossible. Hypopomus discriminates the polarity of a single period sinusoidal stimulus under scanning conditions but does not discriminate under clamped conditions. Hypopomus gives the strongest response to single period sine waves of 670 Hz and weaker responses to sinusoids of lower and higher frequencies. Free-running and phase-locked stimuli evoke similar responses. Under free-running conditions, Hypopomus discriminates a series of EOD-like stimuli that have been phase-shifted by varying amounts, but under phase-locked conditions does not. Scan sampling is presented as a possible waveform recognition mechanism for pulse-discharging gymnotiform fishes.