Data on the reproductive biology of the genus Hippolyte are available from studies of Hippolyte inermis. These findings support the idea that these species are protandric hermaphrodites, exhibiting sex reversal. However, recent studies of this and other species, such as Hippolyte obliquimanus and Hippolyte williamsi, have not reported sex reversal and suggest a gonochoric condition. In the present study, histological analyses were conducted of the ovaries, testes, oviducts, vasa deferentia and the development of the male appendage. The results of this study show that the Hippolyte niezabitowskii population from southern Spain should also be viewed as a gonochoric species. The study found no evidence of sex reversals. These conclusions are also supported by information on the structure of the population and by the results of studies of the male appendage. The population structure involves different size distributions of males and of females. Size classes 2-14 exhibit substantial overlap. No evidence of sex reversal was obtained from the study of the male appendage.