Problems have been encountered in maintaining effective concentrations of the lampricides 3-trifluoromethyl-4-nitrophenol (TFM) and 5,2a??-dichloro-4a??-nitrosalicylanilide (Bayer 73) during treatments of certain Great Lakes tributaries. Concentrations of Bayer 73 decreased by more than 80% in a portion of the Ford River, Michigan, during treatment in 1980. Adsorption of Bayer 73 on sediments was hypothesized as the primary mechanism of this excessive loss. Subsequent laboratory studies demonstrated that lampricides are adsorbed by silt-type sediments that are high in organic content, including those collected from the treated portion of the Ford River. Un-ionized lampricides (acidic solution) were more readily adsorbed than ionized forms (basic solution). Adsorption onto sediments was proportionally greater, and desorption proportionally less, for Bayer 73 than for TFM. When Ford River sediments were mixed with lampricide-free water, less than 10% of the adsorbed Bayer 73, but more than 60% of the TFM, was released. The extensive adsorption of the lampricides (especially Bayer 73) from solution by silt-type sediments explains much of the loss of effective concentrations in certain streams.