Three species of aquatic invertebrates, sampled at sites in Lake Michigan, were subjected in the laboratory to known sodium chloride and potassium chloride concentrations in aqueous solutions. Lethal levels for each organism were observed and recorded for periods of 48 h. One hundred percent mortalities for NaCl concentrations were calculated from a regression line to be 3735 mg/liter forNais variabilis(Piguet), 8865 mg/liter forCricotopus trifasciaEdwards, and 10,136 mg/liter forHydroptila angustaRoss. KCl concentrations at 204 mg/liter forN. variabilis, 4896 mg/liter forC. trifascia, and 6317 mg/liter forH. angustawere calculated for the 100/ mortality level. The lethal aspects of these chloride salts are discussed.