Examination of 122 specimens of the deep-sea fish, Bathypterois mediterraneus (Teleostei, Chlorophthalmidae), collected in the eastern Mediterranean and ranging from 35–158 mm SL, revealed that organization of the hermaphroditic gonad is first noticable in fish of 60–65 mm SL. At this stage, the two parts of the twin gonad are formed by elongated testicular and ovarial primordia enveloped in a common mesodermal tunic. Maturation of testes and development of eggs are described. The ovary is monocyclic, producing one spawn of approximatly 2400 eggs from the end of September to the beginning of November.