Marcusenius pongolensis (South Africa) and M. altisambesi (Upper Zambezi/Okavango) differ in the sex difference of their electric organ discharges (EODs). It is permanent and graded in the former and seasonally dimorphic in the latter. Four out of six experimental female M. altisambesi gave significantly stronger responses to the long playback EODs of breeding males rather than short EODs, whereas four of seven experimental males responded in the opposite way (with one exception in association time). When re-tested after a year, the female preference for long EODs had vanished. Both sexes discriminated between playback EODs as either long or short (unlike M. pongolensis), with the cutoff between 1420 and 2470 mu s (female EOD average, 350 mu s). Exposure to social stimuli from tank neighbours of both sexes did not induce growth of EOD duration in males (unlike M. pongolensis). We suggest that in M. altisambesi the sexually dimorphic male EOD has evolved under female (inter-sexual) selection (in M. pongolensis, both inter-and intra-sexual selection); that there is no evidence for male-male competition (intra-sexual selection, present in M. pongolensis); that ecological differences between a floodplain (M. altisambesi) and a more riverine (M. pongolensis) species may be at the origin of the differentiation.