In adult males of the South African weakly electric bulldog fish, Marcusenius pongolensis, the duration of the electric organ discharge ( EOD) increases with body size over lifetime. Although there is experimental support for intrasexual selection ( male-male competition) having shaped the males' EOD pulse duration in evolution, nothing is known about intersexual selection, such as female choice. Playback of 25 natural male EODs of pulse duration varying from 320 mu s ( close to the average female value) to 716 mu s, to eight female experimental subjects elicited approach, head butts and circling behaviour. The rate of head butts on the dipole electrode model increased significantly with stimulus pulse duration in seven out of eight experimental subjects. In ten experimental female subjects we contrasted the shortest playback pulse with simultaneous playback of one of four longer ones ( 424, 524, 628 and 716 mu s). Pooled responses for all experimental subjects were stronger for the dipole playing back the longer pulse in a pulse pair. The difference in the number of head butts ( Delta head butts) that were dealt the two dipoles per 60 s test session increased significantly with the difference in pulse duration ( Delta pulse duration). The increase followed a significant linear trend ( P<0.0001). Similar results were obtained for Delta association time, Delta circles with head butts, and Delta circles without head butts. These results suggest that a male's reproductive success is enhanced by longer, i.e. more attractive EODs, and that both intra- and intersexual selection must have played a significant role in shaping the EOD of male M. pongolensis.