This paper evaluates the role of electric cues in the sex recognition behavior of an African electric fish, Polimyrus isidori, during courtship behavior observed in the laboratory. I examined the importance of the electric organ discharge (EOD: waveform of the stereotyped electric organ discharge) and the sequence of pulse intervals (SPI: temporal pattern formed by sequences inter-EOD intervals) experimentally by presenting caged fishes, and electric playbacks (models) to male residents. In this species males court females with sounds, and I used this sonic behavior to measure sex recognition. Resident males produced many more grunt sounds when females were introduced onto their territories, compared with caged males, revealing that residents discriminated the sex of the caged fish. Response rates were correlated with characteristic patterns in the SPI of the introduced fish, suggesting that discrimination was based on electric cues. Moreover, I noted several significant sex differences in the SPIs of caged fishes. Residents also discriminated between electric signals from female and male fishes, played to them through electrodes, indicating that electric cues alone were sufficient for sex recognition. An ANOVA was used to ascertain the role of the SPI and the EOD, by analyzing responses to natural female and male models, as well as responses to male EODs combined with female SPIs, and to female EODs combined with male SPIs (hybrid models). I found that discriminating residents used SPI cues alone, with no dependence on the EOD. Taken together with new data showing that females generate a highly regular SPI during natural courtship encounters with singing males, these result show that SPIs are important in sex recognition in P. isidori.