The tropical South American teleost Eigenmannia lineata showed a spontaneous preference for the female type, compared with the male type, of its sexually dimorphic, weak-electric organ discharge (EOD). Female and male EODs differ in waveform and harmonic content. An isolated fish was simultaneously stimulated with digitally synthesized "natural" male and female EODs of equal peak-to-peak amplitudes, at _+35 Hz frequency
difference centered on its stable resting discharge frequency. The stimulus dipoles were arranged symmetrically to the right and left of the fish's hiding place. All stimulus conditions were
permuted at random sequence. Among 11 fish tested, 8 showed a statistically significant preference for one stimulus, the female type, as measured by the amount of time a fish spent close to a stimulus dipole (P< 0.05 in each fish, two-tailed). Thus
female EODs rather than male EODs were more attractive to adult and juvenile fish of both sexes. It was also concluded that E. lineata is capable of discriminating female from male EODs by a complex sensory capacity requiring neither amplitude nor frequency cues. The EOD waveform changed very little within the ecological range of water conductivities (approximately 10-100 gS.cm 1); the P/N-ratio (a waveform character based on zerocrossing intervals) depended only weakly, but significantly, on conductivity (negative correlation in all four fish). Also, the effect of temperature on EOD waveform was very weak: Q~o-values of the P/N-ratio were below but close to I in all fish (27_+5 ~ C). Thus, it can be concluded that the EOD waveform is remarkably stable within widely changing conditions - even beyond the variation found in the field - and is therefore potentially useful as a social cue.