Studies were conducted to assess the selective toxicity of an aqueous phenol, 3-trifluoromethyl-4-nitrophenol (TFM) in sea lamprey, channel catfish, rainbow trout and bluegill sunfish. The ability to bio transform TFM in the different species was investigated by determining UDP-glucuronyl transferase (UDPGT) kinetics in vitro from hepatic microsomal preparations. Maximal velocity (Vmax, nmol/min mg protein) for UDPGT activity toward TFM was significantly greater (p<0.05) in bluegill (1.52), trout (1.82) and catfish (1.46), than in lamprey (0.68). UDPGT activity toward another phenolic substrate, pnitrophenol (pNP), was negligible for lamprey, however significant activities were measured for trout pNp-uDpGT(vmax=1.30). An analysis of Vmax/Km ratios (a measure of enzyme efficiency, nmol/min mg µm substrate) using TFM as a substrate, indicated that the efficiency of UDPGT activities in lamprey (0.0037), catfish (0.0107), trout (0.0189) and bluegill (0.0281) appeared to be greatly influenced by binding affinity (Km), rather than the VmaX of the reaction. These calculated ratios appear progressively lower in species which were previously reported to be more sensitive (i.e., lower LC₅₀S) to aqueous TFM. Results of this study suggest that UDPGT activity in vitro may serve as a useful indicator of fish sensitivity to aqueous phenols.