SUMMARYThis article shows that differences in the waveforms of the electric organ discharges (EODs) from two taxa are due to the different responsiveness of their electric organs (EOs) to their previous activity (auto-excitability). We compared Gymnotus omarorum endemic to Uruguay (35° South, near a big estuary), which has four components in the head to tail electric field(V1 to V4), with Gymnotus sp. endemic to the south of Brazil, Paraguay and Argentinean Mesopotamia (25° South, inland),which shows a fifth component in addition to the others (V5). We found that: (a) the innervation pattern of the electrocytes, (b) the three earlier, neurally driven, EOD components (V1 to V3), and(c) their remnants after curarisation were almost identical in the two taxa. The equivalent electromotive forces of late components (V4 and V5) increased consistently as a function of the external current associated with the preceding component and were abolished by partial curarisation in both taxa. Taken together these data suggest that these components are originated in the responses of the electrocytes to longitudinal currents through the EO. By using a differential load procedure we showed that V4 in G. omarorum responded to experimental changes in its excitation current with larger amplitude variations than V4 in Gymnotus sp. We conclude that the differences in the EOD phenotype of the two studied taxa are due to the different EO auto-excitability. This, in turn, is caused either by the different expression of a genetic repertoire of conductance in the electrocyte membrane or in the wall of the tubes forming the EO.