Description:    The present study analyzes the acute and behavioral toxicity of two commercial formulations of endosulfan and atrazine (Gesaprim 90 WDG® and Zebra Ciagro®, respectively) on the cyclopoid copepod Mesocyclops longisetus . The studied behavior was the “escape ability” because of its ecological importance in natural predator–prey interactions. This was investigated using two experimental designs: (1) a simulated predator (applying a hydraulic device) and (2) a real one (the zooplanktophagous fish Cnesterodon decemmaculatus ). Both pesticides resulted highly toxic to adults and nauplii at even relatively low concentrations and similar to those found in field studies. Copepods’ survival was not only directly affected but also indirectly through altering their escape behavior, which may have increased their vulnerability to predation. The escape ability, measured with the simulated predator was stimulated early (up to 6 h of exposition) but inhibited later (after 24 h of exposition). The predation experiments with the real predator were in accordance with these results. The comparison of both experimental designs corroborates the effectiveness of the hydraulic mechanism as a testing method. Content Type Journal Article Pages 1-9 DOI 10.1007/s11270-012-1375-2 Authors María Florencia Gutierrez, Instituto Nacional de Limnología (CONICET-UNL), Ciudad Universitaria, 3000 Santa Fe, Argentina Ana María Gagneten, Facultad de Humanidades y Ciencias (UNL), Ciudad Universitaria, 3000 Santa Fe, Argentina Juan Cesar Paggi, Instituto Nacional de Limnología (CONICET-UNL), Ciudad Universitaria, 3000 Santa Fe, Argentina Journal Water, Air, & Soil Pollution Online ISSN 1573-2932 Print ISSN 0049-6979 Journal Volume Volume 224 Journal Issue Volume 224, Number 1