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¹ Universidad Autónoma de Aguascalientes, Centro de Ciencias Básicas, Departamento de Química. Avenida Universidad 940, Aguascalientes, Ags. México, C.P. 20100 DOI : 10.4194/1303-2712-v19_7_08 Viewed : 4041 - Downloaded : 3678 A comparison of acute toxicity (LC50 values) among nine toxicants: a) six metals: Cd, Cr, Cu, Hg, Pb, Ti, and b) three organics: benzene, ethyl acetate, and toluene, between the exotic cladoceran species Daphnia magna Straus 1820 and two native strains of freshwater cladoceran species: Daphnia pulex Leydig, 1860 and Simocephalus vetulus (O. F. Müller, 1776) was performed. We hypothesized that the exotic species would be less sensitive than native species. Our hypothesis was fulfilled. Daphnia magna was less sensitive than native species to eight of the nine compounds we analyzed. Results suggest that native species are better adjusted to local environmental conditions, and are more reliable as bioindicators of potential effects of toxicants on aquatic biota. Although the use of D. magna is recommended because of its ample toxicity database, some researchers propose the use of native species for toxicity tests. Therefore, to propose a native species to be considered as a test organism for official toxicity test for tropical countries, there is a need to increase the database of toxicants and to compare the sensitivities of several classes of toxicants with a model organism whose sensitivity to a broad variety of toxicants is well known, like D. magna. Keywords : Aquatic Toxicology, Cladocera, Environmental Legislation, Environmental Toxicology, Metal Toxicity