Abstract This study was conducted at the request of Exxon Corporation to determine the static acute toxicity of MRD-80-3, to the water flea, Daphnia magna. The test was performed by Union Carbide Environmental Services (UCCES) at its toxicity laboratory in Tarrytown, New York. The test material is a clear liquid, insoluble in water. Daphnids occur in nearly every type of freshwater habitat and occupy a major place in the aquatic food web, serving as a principal food for many species of fish. Daphnia magna, because of its wide geographic distribution, temperature requirements and importance in the food web, is recommended as a bioassay organism by the Committee on Methods for Toxicity Tests with Aquatic Organisms (1975). Stock cultures are maintained at 19-21 degrees C in 350 liter tanks. Before starting the test, adults with full brood chambers were isolated into UCCES well waters; next morning the newly released instars, less than 20 hours old, were carefully removed with wide bore pipette and transferred to a separate holding vessel. One hour before the test they were fed, and as food was administered thereafter. Dilution water for the test was obtained from a well on the Tarrytown site, since first instar Daphnide readily survived in water from the same source.