ABSTRACT: The population biology of Pseudorchestoidea brasiliensis (Dana, 1853) (Amphipoda: Talitridae) was studied by monthly sampling from June 1993 through May 1995 at Prainha Beach, Brazil. Three transects were established, each with 5 levels parallel tothe waterline. From each level, 4 replicates were taken with a 0.04 m² sampler. A stratified distribution was observed, with individuals 3 to 8 mm in length concentrated at the lower levels, while 9 to 12 mm individuals were distributed fartherfrom the waterline. Highest population densities were observed in winter (August 1993 and July 1994) and summer (February 1994 and 1995). Ovigerous females and juveniles were observed during the entire sampling period, indicating continuous reproductionof the population. The sex ratio was 1.45 for females. Mean brood size was 2.8 eggs and mean egg volume was 0.04 mm³. The production of small eggs might be linked to small size at maturation of the females, which could increase the populationgrowth rate. Growth and mortality rates were greater for males than females. Annual production was about 0.30 g AFDW m^-2 yr^-1 and the P/B ratio varied from 2.16 to 2.3, while life span varied between 12 and 21 mo.