The lampricidal effects of 3-trifluoromethyl-4-nitrophenol (TFM), 2',5-dichloro-4'nitrosalicylanilide (Bayer 73), and a 98:2 mixture of the two (TFM:2B) were tested against larvae of the sea lamprey (Petromyzon marinus) under controlled laboratory conditions. The lampricides were tested in water at temperatures of 7,12, and 17 C; total hardnesses of 44,170, and 300 mg/l as CaCOa; and pH's of 6.5, 7.5, and 8.5. Temperature had little influence on the toxicity of the lampricides, but the effect of Bayer 73was slowed in cold water. Water hardness did not significantly influence the activity of the 98:2 mixture. The toxicities ofTFM, Bayer 73, and TFM:2B were significantly reduced in water of high pH. Burrowed sea lamprey larvae were less vulnerabletoTFM, Bayer73,andTFM:2Bthanwerefree-swimminglarvae. TFMand TFM:2B were selective for free-swimming lampreys over the nontarget organisms used for comparison, but the margin of safety for nontarget organisms over burrowed sealampreys was narrow.