A herbicidial mixture of diquat and endothall used for control of aquatic macrophytes was evaluated to determine the environmental persistence of the combination and its components and phytotoxicity for egeria (Egeria densa). The tests were conducted in wooden pools illuminated with a 15 hr photoperiod with flourescent lights. Diquat was assayed by a spectrophotometric procedure and endothall was quantified using a gas-liquid chromatographic technique, Persistence of diquat in water was decreased by the addition of plants and sediments to the system; whereas the decline of endothall in water was independent of the presence of plants and sediments at the concentrations tested. No measurable differences existed between the persistence of the mixture under lentic and lotic conditions. The presence of each herbicide in the mixture appeared to decrease the uptake in plants and sediments and increase the persistence of the other herbicide in water. Results of testing the mixture and its components individually under various water hardnesses (32 mg/1 to 460 mg/1) indicate the phytotoxicity of the mixture and diquat decreased as water hardness increased, while the phytotoxicity of endothall remained unchanged. Herbicide concentration decline in water appeared independent of water hardness. While diquat was effective in controlling egeria, endothall was not, indicating that the role of the mixture for treating egeria should be reevaluated. / M.S.