The salinity tolerances of adult females of some relevant species of planktonic copepods were studies in a mangrove region of the estuarine type, at 25º south latitude. Results showed a wide range in the salinity tolerances for the adultfemales of all the species studied. Based on these experiments the following order of salinity tolerance can be proposed: Pseudodiaptomus acutus > Euterpina acutifrons > Acartia lilljeborgi > Oithona ovalis > Centropages furcatus > Temora stylifera. A mechanism for the maintenance of Pseudodiaptomus acutus at certain regions of the area studied is proposed, based on experimental and field results. It seems probable, thus that salinity is an important factor in the distribution of some members of the zooplankton in this region.