The impact of glufosinate-ammonium and bialaphos on the zooplankton community in a small eutrophic lake was investigated using in situ enclosures. Concentrations inducing a 20% reduction in abundance (EC20), as interpolated from best-fit, nonlinear regression analyses were similar for both herbicides and ranged from 0.03 to 0.16 mg/L for various zooplankton taxa. Similarly, median effective concentrations (EC50) estimates ranged from 0.12 to 0.50 mg/L. Thus, toxicity endpoints overlapped and in some cases were well below expected environmental concentrations calculated for accidental direct overspray (1 mg/L) or drift events (0.25 mg/L). Significant concentration-dependent reductions were observed within the first 2 weeks following application and for a number of taxa persisted throughout the observation period (63 d posttreatment). At the highest treatment level, (10 mg/L), negative impacts were still apparent in the year following treatment. The results of this field study, which demonstrate significant negative effects on a variety of zooplankton taxa at environmentally relevant concentrations and relatively slow recovery therefrom, suggest a substantial risk of sustained adverse impacts on the zooplankton communities should these herbicides contaminate shallow lentic ecosystems. Extensive mitigative measures are required to protect such water bodies from potential impacts of phosphinothricin-based herbicides.