Although polyelectrolytes are widely used to control solids in potable water and municipal wastewater, very few studies have been made to assess their toxicity to aquatic animals. Consequently, the authors tested the acute toxicity of several polyelectrolytes to rainbow trout, lake trout, a mysid, a copepod and a cladoceran in Lake Superior water. They also tested the chronic toxicity of three of these compounds to Daphnia magna to study possible effects on reproduction. Their results indicated that some of the cationic polyelectrolytes tested were toxic at concentrations that might easily be released into the aquatic environment and cause serious problems to aquatic life.