Mormyrid fish generate electric organ discharges (EODs) continuously for electrocommunication and electrolocation. EOD pulse waveforms are, within species limits, individually specific and usually stable over long periods of time. We conducted playback experiments with the African weakly electric snoutfish, Marcusenius macrolepidotus (South African form), or bulldog fish, originating from the Incomati River System in South Africa. Previous field observations had shown an increase of male, but not female, EOD pulse duration with body size. Therefore, we tested the hypothesis that EOD pulse duration might be of relevance in territorial male-male contests. We determined whether or not aggression scores recorded for males kept in adjacent aquarium compartments depended on playback EOD duration. Playback EODs had been pre-recorded either from familiar or from unfamiliar conspecific males, and varied naturally in duration. As independently confirmed in all five experimental subjects, the attack rate on a dipole model playing back strangers' EODs from the territorial boundary increased significantly with stimulus EOD pulse duration (that varied from 329 μ s to 975 μ s in our field sample of playback EODs). This result was confirmed when one specific EOD, the duration of which was artificially decreased or increased, was used for playback, showing that in male M. macrolepidotus EODs of so long duration signal a greater threat than EODs of shorter duration. Therefore, EOD duration may be relevant for males in the assessment of their rivals' size and fighting potential. Moreover, experimental subjects confronted with familiar playback EODs showed a significantly lower rate of attack than expected for unfamiliar EODs of the same duration. An individual familiar neighbour's EOD evoked a significantly weaker attack rate when played back from the regular rather than from the opposite territorial boundary. These observations are compatible with the 'dear enemy' hypothesis.