The adverse effects of arsenic and fluoride exposure on six groups of freshwater invertebrates were investigated. Acute toxicity tests (48-h) with arsenic trioxide (As2NO3) resulted in the following pattern of sensitivity: Daphnia magna 24-h-old = Brachionus patulus 72-h-old = Daphnia. cf. prolata, 21-d-old = D. magna 5-d-old > Heterocypris incongruens juvenile instars > Culex sp. Heterocypris juv. incongruens instars were the second group more tolerant to arsenic and the second group that bioconcentrates arsenic the least. In contrast, invertebrates exposed to sodium fluoride (NaF), showed a different pattern of sensitivity: H. incongruens instars > B. patulus = D. magna 24-h-old > D. cf. prolata 21-d-old = Culex sp. = D. magna 5-d-old. Our results suggest that all species tested might be considered good model tests organisms for As toxicity except H. incongruens. The rotifer B. patulus did not accumulate either arsenic or fluoride; and its sensitivity was intermediate for both toxicants. In contrast, D. cf. prolata accumulated more fluoride and was also (together with 5-d-old D. magna) the most tolerant to fluorine. In the case of arsenic, 5-d-old D. magna were the organisms with highest accumulation rates, but their sensitivity was similar to all other species (except for Culex sp. and H. incongruens). Interestingly, H. incongruens juv. instars have low sensitivity to As but are the most sensitive species to fluoride exposure. These results point out to the need of consider several invertebrate species as model organisms for environmental protection of particular ecosystems, or that some freshwater species have the potential to be used as fluorine bioaccumulators in remediation processes.