This study examines the utility of plant peroxidase (POD) activity as a biochemical indicator of chemical stress in the aquatic plant Hydrilla verticillata (Royle). Plants were raised in 10% Hoagland's solution augmented with 200 mg NaHCO3/L. The plants were exposed to concentrations of exponential increment of Cu2+ or the sulfonylurea herbicide, sulfometuron methyl, (methyl 2-[[[[(4,6-dimethyl-2-pyrimidinyl) amino]-carbonyl] amino] sulfonyl] benzoate), also known by the trade name OustR. After seven days of exposure the plants were harvested, macerated, and the POD extracted with salt solution. POD was measured by a spectrophotometric assay. There was a significant increase in POD activity after exposure to sublethal concentrations of Cu2+ or Oust. This increase in POD activity was dose dependant and concomitant with a decrease in root and shoot elongation. Plants exposed to 1 mg/L Cu2+ resulted in a 1.75-fold increase over the controls in shoot POD activity. Plants treated with 1 mg/L Oust resulted in a 1.5-fold increase in shoot POD activity over the controls, yet no necrosis was visible after seven days. These results indicate that measuring the saltextractable POD activity has great potential as a biochemical indicator of sublethal stress in the aquatic plant H. verticillata.