Comparative data on the reproduction in captivity and ontogenetic development of six of mormyrid species and seven gymnotiform species are reported. Mormyrid fishes: egg diameter ranged from 1.8 (Petroephalus soudanensis) to 3.0 mm (Hippopotamyrus pictus. Campylomormyrus phantasticus); fecundity (egg number per spawning) from 28 to 215 (Pollimyrus isidori) and 121 to 1662 (Campylomormyrus cassaicus); spawning intervals from 5 to 20 days (Pollimyrus isidori) and 15 to 80 days (Campylomormyrus cassaicus). Pollimyrus isidori is the only species exhibiting parental care (in the male sex). Gymnotiform species: egg diameter ranged from 1.7 (Brachyhypopomus pinnicaudatus) to 3.0 mm (Sternopygus macrurus, Rhamphichthys sp., Gymnotus carapo); fecundity (egg number per spawning) from 1 to 105 (Apteronotus leptorhynchus) and from approximately 500 to approximately 1000 (Rhamphichthys sp.); spawning intervals from 2 to 5 days (Eigenmannia lineata) and from 20 to 41 days (Rhamphichthys sp.). Sternopyguls macrurus exhibited parental care (in the male sex) by guarding the eggs, whereas Gymnotus carapo revealed to be a mouthbreeder (guarding free embryos) in the male sex. Gonad maturation could be provoked by increase of conductivity alone in all mormyrid species tested and in several gymnotiform species. Four different stages of morphological development (hatchlings, larvae at beginning of exogenous feeding, juveniles, adults) are described in both taxa. The reproductive strategies of the gymnotiforms are considered more diverse than those of the mormyrids.