The Spanish toothcarp Aphanius iberus is an endangered species which inhabits small rivers, creeks, salt marshes and marine salt pans in the Mediterranean coast of Spain. No differences in weights were observed among females or males taken from different environments. Analyses of the morphology of the gonads and the gametogenesis were performed in fish taken from different environments by comparing gamete development in females and in males and gonadal cell proliferation in the testis. A high degree of plasticity was observed in the gonad morphology of A. iberus. Females possess two ovaries which show non-restricted oogenesis with all germ cell stages within the same ovigerous lamellae, while males possess gonads without any clear division with the typical restricted pattern observed in cyprinodontid fish. Some females and males showed asymetrically developed gonads. Proliferation of germ cells in testis is located only in the periphery of the gonads corresponding with primary and secondary spermatogonia. Salinity did not influence gonad plasticity or the appearance of mature germ cells in either females or males. This is the first study to provide a microscopic description of oogenesis and spermatogenesis in A. iberus at extreme different environmental conditions.