Data from this study will be used to extend the current pH/ alkalinity sea lamprey MLC prediction chart used to set 3-trifluoromethyl-4-nitrophenol (TFM) concentrations for treating streams to control sea lamprey populations in the Great Lakes. The current chart predicts levels of TFM required to kill 99.9% of larval sea lampreys in water with alkalinities up to 260 mg/L. Some streams in lower Michigan, however, exceed the current 260 mg/L alkalinity threshold. Treatment managers have requested an extension of the chart to an alkalinity of 300 mg/L for pHs between 8.0 and 8.5. Exposures were conducted with TFM in water at pH 8.0 and 8.5 at alkalinities 260 and 300. Differences in TFM toxicity between the two alkalinities at the two pHs tested will be used to develop an extension in the prediction of sea lamprey MLCs at water alkalinities up to 300 mg/L.