Trichloroacetic acid (TCA) has been detected in rain, snow, and river samples throughout the world. It may enter into natural water systems via herbicide use, as a by-product of water disinfection, from emissions of spent bleach liquor of kraft pulp mills, and as a natural fungal product. This compound is phytotoxic and likely to accumulate in aquatic environments. A study to assess the fate of TCA in semi-natural aquatic environments and the toxicity of TCA to rooted aquatic macrophytes was conducted. The experiment involved exposing three replicate 12000 l aquatic microcosms at the University of Guelph Microcosm Facility to 0.05, 0.5, 3, and 10 mg/l of TCA for 35 days in a one-way analysis of variance design. Each microcosm was stocked with 14 individual 5 cm apical shoots of Myriophyllum spicatum and M. sibiricum. The plants were sampled at regular intervals and assessed for the somatic endpoints of plant length, root growth, number of nodes and wet and dry mass and the biochemical endpoints of chlorophyll-a and chlorophyll-b, carotenoid content, and citric acid levels. TCA half-lives in the microcosms ranged from 190 to 296 h depending on the initial concentration of TCA. Myriophyllum spp. results indicate that while there were some statistically significant differences from controls, there were no biologically significant effects of TCA for any of the endpoints examined. These data suggest that TCA does not pose a significant risk to these macrophytes up to 10 mg/l, which typically exceeds environmentally relevant concentrations by several orders of magnitude.