Metolachlor is a preplant, preemergent herbicide applied to corn and soybean fields. Agricultural runoff after application can cause the herbicide to enter into natural waters. The objective of this study was to determine the effect of river and wetland sediments on the toxicity of metolachlor. Sediments from a river and a wetland (separately) were mixed with Ottawa sand (0, 25, 50, 75, and 100%). Metolachlor (in water) and sediment were mixed in an orbital shaker for 8 hr; the mixture was centrifuged, and the supernatant liquid was tested for toxicity (EC50%) using the marine luminescent bacteria Vibrio fischeri (Microtox). The toxicity (EC50%) of metolachlor with the river sediment was 64.61. Metolachlor after interaction with the wetland sediment demonstrated no toxicity possibly due to increased adsorption on the higher amount of organic matter (10 times) and clay (3.5 times) present in the wetland sediment than the river sediment.