Technical Abstract: Aquatic vegetation has been shown to improve water quality by trapping and processing contaminants such as pesticides, nutrients and sediments. Currently there is little information regarding effects of pesticide and nutrient mixtures on aquatic biota in these systems and the influence aquatic vegetation may have under these conditions. The purpose of this study was to measure the toxicity of a pesticide-nutrient mixture in vegetated and non-vegetated wetland mesocosms using Hyalella azteca 48 h whole effluent bioassays. Wetland mesocosm 63 m L x 30 m W was evenly divided into two sections. One section contained no emergent vegetation and the other was naturally vegetated with Eleocharis obtusa and Carex lurida. Both sections were amended with a mixture of diazinon insecticide (37.62 g a.i.), permethrin insecticide (0.4104 g a.i.), triple super phosphate (270 g P), and sodium nitrate (886.5 g N). Aqueous samples were collected at inflow, middle, and outflow points within each section 5 h, 24 h, 72 h, 7 d, 14 d, and 21 d after amendment began. Diazinon concentrations (µg/L) in the vegetated section ranged from 80.9-107.2 at 5 h to 2.2-4.1 at 21 d; in the non-vegetated section 104.3-121.9 at 5 h to 3.1-3.3 at 21 d. Permethrin concentrations (µg/L) in the vegetated section ranged from 0.8-6.6 at 5 h to 0-0.1 at 21 d; in the non-vegetated section 4.0-7.7 at 5 h to 0-0.1 at 21 d. Filterable orthophosphate concentrations (mg/L) in the vegetated section ranged from 0.71-1.79 at 5 h to 0.05-0.15 at 21 d; in the non-vegetated section 0.73-1.48 at 5 h to 0-0.03 at 21 d. Dissolved inorganic nitrogen concentrations (mg/L) in the vegetated section ranged from 0.89-2.71 at 5 h to 0.17-0.18 at 21 d; in the non-vegetated section 1.30-2.43 at 5 h to 0.19-0.21 at 21 d. Estimated 50 percent lethal effluent effects dilutions, LF50s (%) at 5 h were 0.20, 0.61, and 2.05 at inflow, middle and outflow in the vegetated section and 0.10, 0.34, and 0.16 at inflow, middle and outflow in the non-vegetated section. While toxicity consistently decreased over time, LF50s were similar among all points within both sections from 24 h to 14 d. No toxicity was observed at any point in either vegetated or non-vegetated section by 21 d after amendment. Results of this study suggest that the effects of vegetation versus no vegetation on pesticide toxicity are not evident after 5 h with the addition of nitrogen and phosphorus.