A study of the aquatic fate of the triethylamine salt of triclopyr (3,5,6-trichloro-2pyridinyloxyacetic acid) was conducted in three bays of Lake Minnetonka, Minnesota. Triclopyr is under review by the US Environmental Protection Agency as a selective aquatic herbicide. The primary purpose of this study was to determine dissipation rates of the parent active ingredient, triclopyr, and its major metabolites, 3,5,6-trichloropyridinol (TCP) and 3,5,6-trichloro-2-methoxypyridine (TMP) in selected matrices including water, sediment, plants, ยฎnยฎsh and shellยฎsh. Two 6.5ha plots dominated by the weedy species Eurasian watermilfoil (Myriophyllum spicatum L) were treated with triclopyr-triethylammonum at a rate of 2.5 mg AE liter ร1 (2.5 ppm) on 21ยฑ23 June 1994. A third 6.5-ha plot was established as an untreated reference. Water and sediment samples were collected from within the plots and at selected locations up to 1600 m outside of the plots through six weeks post-treatment for chemical residue analysis. In addition, residue samples were collected from the target and non-target plants and other non-target matrices, including game and rough ยฎsh, clams and crayยฎsh. All test animals were sequestered in cages located in the center of each plot and samples were collected through four weeks post-treatment. Half-lives for dissipation of triclopyr and TCP in water ranged from 3.7 to 4.7 days and from 4.2 to 7.9 days, respectively, with trace amounts of TMP found. Peak triclopyr sediment values ranged from 257 to 335 ng gram ร1 , with a mean half-life of 5.4 days, while peak TCP sediment levels ranged from 27 to 65 ng gram ร1 (mean halfรlife = 11.0 days). Trace levels of TMP were detected at one treatment site at one sampling event. Triclopyr and TCP accumulated and cleared from animal tissues proportionately to concentrations in the water (triclopyr dissipation half-lives 11 days, TCP 14 days). TMP levels were two to three times higher than those of the other compounds, particularly in visceral tissue. In all cases, residues of these compounds were higher in the inedible portions of the animals, and were usually higher in bottom-feeding ยฎsh species.