Abstract:The neotropical halophilic cladocerans Daphnia menucoensis and Moina eugeniae are common in shallow lakes of semi-arid central and northern Patagonia in Argentina. In the present study, we determined the tolerance of neonates of both species to salinity and different concentrations of Cl -and SO 4 2-. However, the responses differed according to the demineralized water or groundwater employed to dissolve the salts. On the other hand, survival was lower when using analytical grade reagents instead of natural salts. As bioassays should be developed to determine the chronic physiological responses of both species, the objectives of this study were to compare three culture media and test the hypothesis that natural salts dissolved in aquifer water is the optimal combination for breeding. Acute bioassays were conducted with neonates of both species. To compare the media, NaCl solutions (5, 10, 15, 20 and 25 g.L -1 ) were prepared using demineralized water, aquifer water and EPA medium. For the second objective, solutions of equal concentrations were prepared with salts obtained from the natural environment, previously sterilized, and dissolved in phreatic water. Survival was higher in the aquifer water, whose chemical composition was relatively more representative of the lakes inhabited by both species. It was almost complete at 20 and 15 g.L -1 for D. menucoensis and M. eugeniae, respectively. Lower survival of D. menucoensis was found in EPA medium, although it was developed for breeding other species of Daphnia. In the second series of bioassays, survival was almost complete until 20 g.L -1 (D. menucoensis) and 25 g.L -1 (M. eugeniae). The combination natural salts/aquifer water was adequate to assess the tolerance of both cladocerans. However, the former was more important, since the results were consistent with previous studies when these salts were dissolved in demineralized water. In addition, survival was much higher than when we used salts of analytical grade.