Salinity is a serious threat to freshwater ecosystems, particularly those near coastal areas. An increase in salinity produces drastic changes in community structure of freshwaters, sometimes in an irreversible fashion. Thus, freshwater species must cope with salinity stress in a manner proportional to their degree of tolerance. Bearing this in mind, we studied the acute and chronic effects of different salinity concentrations in two species of cladocerans: Daphnia magna Straus, a standard test organism, and Daphnia longispina O. F. Müller, an autochthonous species. Salinity experiments were based on successive dilutions of a stock solution of NaCl in a synthetic medium. The results showed that D. magna is more tolerant than D. longispina, both in acute (EC50 5.9 and 2.9 g/L, respectively) and chronic (EC50 5.0 and 2.2 g/L, correspondingly) exposures. In the chronic exposure, salinity caused a significant reduction in fecundity and a developmental delay (increase in age at first reproduction), as well as a decrease in the growth rate of daphnids. However, these effects were mainly observed at salinity concentrations where mortality occurred.