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The evaluation of ecotoxicity of mosquito larvicidal agents (such as the water-soluble lectin from Moringa oleifera seeds, WSMoL) is an essential step to establish the guidelines for their use. In this sense, this work evaluated the toxicity of WSMoL to Danio rerio embryos and larvae. Embryos were exposed to waterborne WSMoL (0.0125-0.2 mg mL) for 96 h and lethal and sub-lethal effects were observed every 24 h. In the bioassays with larvae, the individuals were exposed to the WSMoL (0.025-0.2 mg mL), mortality was recorded daily, and larval swimming velocities were analyzed after 72 h and 168 h of exposure. Additionally, acetylcholinesterase (AChE) activity of larvae was determined after 168 h of exposure. WSMoL LC values to embryos were 0.190, 0.133 and 0.049 mg mL after 48, 72 and 96 h, respectively. No toxic endpoint was observed after exposure for 24 h. In addition, hatching was delayed and larval length at 96 h was reduced compared to the control. WSMoL LC to larvae were 0.21 and 0.135 mg mL, after 24 h and 96 h, respectively. Larvae exposed to 0.1 and 0.2 mg mL showed a decrease in swimming speed and a significant reduction in AChE activity. In conclusion, WSMoL at waterborne concentrations needed for its use as a larvicide to A. aegypti causes lethal and sublethal effects to zebrafish embryos and larvae. Therefore, its use in waterbodies where there are non-target organisms is not recommended.