the population structure and reproductive aspects of atherinella brasiliensis (quoy & gaimard, 1825) had been analyzed monthly from may 2000 to april 2001 in eight points of shallow areas of the estuarine complex paranaguá. the environment analyzes of the parameters ph, dissolved oxygen, transparency and salinity of the water suggest the existence of an increasing environment gradient in the direction north-south highway of the estuary. the temperature of the water evidenced the seasonal variation in the studied system, revealing related with the reproductive process. the peak of reproductive activity was determined for females and male, respectively, in the months of october and september, period where if it verified the ratio of two females for a male. the predominance of young individuals in the estuary occurred in the end of the spring and during the summer, after the reproductive period. the seasonal analyses of length and mass suggest the existence of a cyclical standard of growth to the long of the year, which reflects the reproductive period and the entrance of young individuals in the environment. thus, it is concluded that a. brasiliensis is a resident estuarine species, revealing widely distributed in the estuarine shallow areas, being able to be considered key in evaluations of environmental impact and/or action of handling plans and conservation in estuarine environments.