The test organism was the freshwater crustacean, Daphnia magna Straus, obtained from continuous laboratory cultures. The stock cultures of Daphnia were maintained in a reconstituted water medium, identical to the test dilution water, at a temperature of 20 + or - 2 deg C with a photoperiod of 16 hours light; 8 hours dark. The cultures were fed a defined diet of algae Chlorella vulgaris and 'Frippak Booster' (a commercially available microencapsulated diet). Culture conditions were such that the Daphnia reproduction was by diploid parthenogenesis.