The granular formulation of Bayluscide [Bayluscide 3.2% Granular Sea Lamprey Larvicide, granular Bayluscide (gB)] is applied in lentic and lotic systems to survey (assessment) and kill (treatment) larval sea lampreys (Petromyzon marinus) in the Great Lakes basin. Granules are spread on the water surface, settle to the sediment surface, and dissolve. The potential risk of niclosamide exposure [5 Chloro-N-(2-chloro-4-nitrophenyl)-2-hydroxybenzamide], the active ingredient of gB, to non-target organisms located downstream of survey plots, is a concern of partner agencies (State-level Natural Resource Departments, U.S. Fish and Wildlife Service’s Ecological Service, Fisheries and Oceans Canada Species at Risk Branch). Temporal and spatial distribution of niclosamide in the water column and sediment was evaluated in and downstream of five larval survey plots in two rivers following the application of gB. Water samples were collected at 0.25, 2, 4, 6, 8, and 24 h from 3 depths in the water column (10 cm above the sediment, ½ water column depth, water surface) at three locations inside each survey plot, and 1 meter upstream from three sediment sample grids positioned 10, 30 and 100 m downstream. Sediment samples were collected from inside the grids at 0.25, 2, 4, 6, 8, and 24 h, and from inside the survey plots, 8 and 24 h after gB application. Niclosamide was detected in the sediment and water at all sample locations. From 2 to 24 h after application, average water concentrations 1) varied between study sites, 2) decreased from the survey plots to 100 m downstream, 3) varied by depth in the water column, and 4) decreased over time. Average sediment concentrations varied with distance downstream and time post application, but not by study site or river. Data suggests there would be negligible risk to non-target organisms downstream of a gB survey plot based on low niclosamide concentrations measured in the water and sediment. The depletion rate of niclosamide was also evaluated in St. Clair River sediment dosed at the field application rate. Niclosamide concentration decreased at a rate of 2.28% per hour over the 24 hours measured, equating to a half-life of 1.27 days. This indicates the length of time an organism in the sediment in a survey plot might be exposed.