The effects of androgens and estrogen on the external morphology and electric organ discharge (EOD) waveform in Gnathonemus petersii, a weakly discharging electric fish, were investigated. Following preimplant data collection, juvenile and adult fish were gonadectomized and implanted with silastic capsules containing either high or low doses of testosterone (T), dihydrotestosterone (DHT), estradiol-17 beta (E2), or cholesterol. One group of fish was treated with high doses of DHT + E2. Radioimmunoassays revealed that low-dose implants resulted in plasma T levels comparable to and high-dose implants about sixfold greater than those found in adult males imported during breeding season. High-dose E2 implants resulted in higher plasma E2 levels in adults than those in juveniles. At either dose, both androgens induced male-like indentations in the dorsal margin of the anal fin of juveniles and adult females by 4 weeks postimplant. Both low and high doses of T decreased the peak power spectrum frequency (PPSF) of Fourier transformations of EODs and increased the durations of phases 2 and 3 of the EOD in juveniles and adults, but the high doses caused more rapid and profound effects. The two doses of T caused opposite effects on the durations of phases 1 and 4 juveniles. The low dose of T decreased the durations of phases 1 and 4, while the high dose increased them. In adults, the high dose of T increased the duration of phase 1, but had inconsistent effects on the duration of phase 4. Total EOD durations were increased by both doses of T in juveniles, while adults showed inconsistent effects possibly due to individual variability in hormone sensitivity. Compared to T, DHT exerted similar, but less dramatic effects on all measures, but only at high doses. E2 significantly increased adult PPSFs, the first such finding in a mormyrid species. E2 had no effects on juvenile PPSFs, or on adult or juvenile EOD phase durations. The effects of DHT + E2 on PPSF and phases 2 and 3 were similar to those of DHT alone. These findings demonstrate quantifiable steroid-dependent plasticity in the durations of individual phases of EODs in an electric fish and are the first to show that the external morphology in Gnathonemus petersii is androgen-dependent. The results are discussed with regard to methodological considerations and hormone studies involving sex differences in EODs reported for this and other species.