Due to their extensive use and their toxicity toward aquatic organisms trialkyltin com-( ) pounds R SnX are among the most problematic water contaminants. The effects of tributyltin chloride 3 ( ) TBTCl on detoxication enzymes were determined in the freshwater macroalga Cladophora sp. and in the freshwater macrophytes Ceratophyllum demersum and Elodea canadensis. Effects on Cladophora sp. as a possible target organism which grows on all substrates, even on ship's hulls, were compared with two nontarget-species C. demersum or E. canadensis. We found an uptake of TBTCl in Cladophora ( ) ( ) sp. and an enhanced metabolism of TBTCl to dibutyltin DBT and monobutyltin MBT in the presence of the algae. Furthermore, there was an elevation of microsomal and soluble glutathione S-transferase ( ) activity and an inhibition of cytochrome P-450 ethoxyresorufin O-deethylase activity in all investigated plants in a dose-dependent manner indicating a significant interaction of TBTCl with nontarget organisms in the aquatic ecosystem. ᮊ