Abstract: Daphnia is a key organism in the food chains of plants, microbes, and animals in aquatic ecosystems. Glyphosate is widely applied to crop fields, orchards, green land, and water, and accounts for about 60% of registered herbicides in China. Cyhalofop-butyl is widely used to control Echinochloa crusgalli in paddy fields. The use of these two herbicides has increased markedly in recent years in China. Therefore, it is important to investigate the toxicity of glyphosate and cyhalofop-butyl to zooplankton to understand their effects on aquatic ecosystems. In this study, Daphnia carinata, a standard bio-monitor of water quality, was cultivated in dechlorinated tap water and fed with Scenedesmus obliquus at (20±0.5) ℃ under light at 1200 lx supplied by fluorescent lamps. Four-day-old D. carinata were incubated in solutions containing glyphosate or cyhalofop-butyl at different concentrations. The acute toxicity of these two herbicides to the movement, survival, and phototaxis of D. carinata was evaluated. Both herbicides were toxic to D. carinata. The semi-lethal concentration (LC₅₀) of glyphosate in 24 h, 48 h, and 96 h was 66.58, 29.60, and 12.33 mg/L, respectively, and that of cyhalofop-butyl was 63.15, 51.91, and 34.41 mg/L, respectively. These dosages represented only 0.15%-0.81% (glyphosate) and 17.21%-31.58% (cyhalofop-butyl) of those applied in the field. Both glyphosate and cyhalofop-butyl had significant adverse effects on the swimming ability and phototaxis index of D. carinata. The concentrations causing significant changes in the phototaxis index at 3 h were 4.59 mg/L for glyphosate and 14.20 mg/L for cyhalofop-butyl; these concentrations corresponded to 0.06% and 7.10% of the field dosage, respectively. These results showed that glyphosate and cyhalofop-butyl are very toxic to daphnia. Therefore, their effects on the aquatic food chain should not be ignored. Changes in the phototaxis of D. carinata in response to herbicides at low concentrations may serve as an index of herbicide bio-toxicity in fresh water.