<p>Pesticides pose a serious threat to non-target organisms and are classified as key contributors to amphibian population decline. Amphibians, known for their sensitivity to environmental changes, serve as valuable bioindicators of environmental health. In this study, we assessed the toxicity of a diquat-based herbicide (DBH) on the bioindicator species <i>Physalaemus cuvieri</i>. Tadpoles were exposed to a broad toxicity gradient of DBH (20, 50, 250, 500, 1000, and 1500 µg L⁻¹) for 336 h. Survival, body mass index (BMI), scaled mass index (SMI), malformations, and swimming activity were evaluated. Exposure to 1000 and 1500 µg L⁻¹ significantly reduced survival. Interestingly, concentrations of 20, 50, 1000, and 1500 µg L⁻¹ led to increases in BMI and SMI. Malformations and alterations in swimming behavior were detected at all concentrations. These findings demonstrate that DBH is highly toxic to <i>P. cuvieri</i> tadpoles, severely impairing their survival, physiology, and behavior. These results have significant environmental implications, as tadpoles play a crucial role in aquatic ecosystems, and this sensitivity to DBH can trigger imbalances in trophic dynamics. Furthermore, amphibians are bioindicators, and the impacts observed here reinforce the need for rigorous monitoring of potentially toxic chemicals in aquatic ecosystems to preserve biodiversity, especially among non-target species.</p>