The study of gonads of more than 100 specimens of eels belonging to 28 species of the family Muraenidae (Teleostei, Pisces) revealed that, although they are uniform in their gross morphology, they are divisible into groups according to cytology and sexuality. One group includes gonochoristic species, such as Muraena helaena, Pseudoechidna brummeri, Echidna delicatula, and numerous species of Gymnothorax, in which males and females are separate. The second group includes protogynous hermaphrodites, such as Muraena pavonina, Gymnomuraena zebra, Gymnothoraxfimbriata, G. margaritophorus, G. gracilicauda, Echidna nebulosa, Uropterygius fasciolatus, and U. polyspilos, in which the male gonad develops as a secondary one within the primary ovary. The third group, comprising all members of the genus Siderea, are simultaneous hermaphrodites in which male and female gonads develop in each individual from the smallest to the largest. The species of the genus Rhinomuraena have been previously shown to be protandric. Thus, in this family of fishes, four fundamentally different strategies of reproduction have been recognized. Various aspects of ontogenesis of these sex characteristics are discussed. T HE morays comprise a large group of fishes, all of which are elongated with fleshy, confluent dorsal, caudal, and anal fins and no paired appendages. The majority of the marine species are found in tropical and subtropical waters, particularly in the shallow, littoral subzones (Bardach et al., 1959; Chave and Randall, 1971; Abrams and Shein, 1986). Within coralreef habitats, these fishes form an important part of the predatory ichthyofauna and include species such as Gymnothoraxjavanicus that attain a length of up to 3 m. Being carnivorous, and especially piscivorous, they play a significant role in the dynamics of numerous prey species. Nevertheless, this group of fishes has been little studied, particularly in comparison with the socalled fresh-water eels of the genus Anguilla (Tesch, 1977). Only recently some short papers have been published on their thermoregulation