The post-marsupial growth, sexual differentiation, fecundity and reproductive biology of Hyale media Dana, 1853 living on Sargassum cymosum C. Agardh, 1820 are described. The growth was continuous througth 12 stages for males and 9 for females. The sexual differentiation occours at 2th or 3th moult and was demonstrated by the enlargment of the gnatopod II propod. Number of eggs increased with the female head length. Observations of courtship behavior, incubation, moult processes, emergence of juveniles and brood caracteristics were made. The precopula courtship continued for two days, the eggs were incubated for six days and the juveniles, that stay until three days in the marsupium, moult every day.
Amphipods; reproduction; growth; fecundity; Sargassum