fish and environmental variables were sampled in a mangrove area from sepetiba bay to assess the role of this system in life cycle of the silverside atherinella brasiliensis (quoy & gaimard, 1824). standardized monthly sampling using beach seines were conducted at five sites, between august, 2002 and july, 2003 in daily (day/night) basis, with environmental variables being taken at each site. the highest numerical abundance was recorded in the intermediate sample area in the mangrove channel and the lowest in the innermost site. the highest weight occurred in the outermost site near to the sea connection. numerical abundance was directly correlated with temperature and salinity, and weight with salinity and dissolved oxygen (p < 0.05). size ranged from 10 to 160 mm tl, with modes of 40-50 mm tl occurring all over the year in all sites. recruitment occurred all year round but peaks (tl = 30 mm) in october-december, 2002 and july, 2003 were recorded afterwards the records of larger fishes (september, 2002 and june, 2003) mainly in the outermost site. the largest fishes concentrated in the sandy substrate of the outermost sites, areas more suitable for a. brasiliensis spawning, with recruits dispersing all over the mangrove areas, which are used as rearing grounds. the affinity for highly structured habitats in mangrove and the capacity to use efficiently semi-closed systems was confirmed for a. brasiliensis in guaratiba mangrove.