The population biology of three species of the family Ampithoidae (Ampithoe ramondi, Cymadusa filosa e Sunampithoe pelagica) associated with Sargassum filipendula at Fortaleza beach, northern coast of Sao Paulo state was investigated from June 2000 to May 2001. The amphipods were identified, counted and separated in size classes through a nested sieve-set. Month hystograms were obtained for juveniles, males and females. The ampithoid sex ratio was determined monthly The reproductive biology of the species was evaluated by the relative frequency of ovigerous females, fecundity, average egg volume and correlation analysis between head lengh of ovigerous females and number of eggs in its brood pouch. Ampithoe ramondi was the most abundant species. The higher densities of the three species occurred in spring and summer. The sex ratio presented a significant difference from 1:1 only for A ramondi (1:1.2; X-2 = 4.37; g.l. = 1; p < 0.05). Ovigerous females were registered for almost all the sampling periods. A positive correlation between the female head lengh and the number of eggs were only registered for A ramondi (r(2) = 0.27; p < 0.05; n = 33). Cymadusa filosa showed the greatest fecundity and egg volume.23412071216