Test specimens of Daphnia magna were obtained from an in-house daphnid culture which ABC Laboratories has maintained since 1977. The primary culture was obtained from the Columbia National Fisheries Research Laboratory (CNFRL), Columbia, Missouri, in 1977. A trace of the daphnid strain indicated that CNFRL acquired their culture from the U.S. Fish & Wildlife Service Fish Control Laboratory, LaCrosse, Wisconsin, in 1960, and they obtained their culture from Pennsylvania State University, in 1954 (1). All daphnids were cultured and tested in a temperature controlled area at 20 (+1).C. The lighting was maintained at 50-70 footcandles on a 16-hour daylight photoperiod, with 30 minute dawn dusk transition periods. The daphnid cultures were maintained in 1.5- liter glass containers. Daphnids were cultured in hard blended water. Hard blended water is a combination of well water and reverse-osmosis water blended to a hardness of 160-180 mg/L as CaCOz. Prior to its use, the hard-blended water is passed through a standard sediment water filter.