Industrial phosphate esters have been widely used for the past 30 years as plasticizers, hydraulic fluids, and gasoline additives because of their fire resistance and performance. A study by the U.S. Environmental Protection Agency indicated that a significant portion of the annual production of all hydraulic fluid was discharged into the environment as a result of leakage. The purpose of the study was to determine the chronic toxicity of Pydraul 50E, an alkyl-aryl phosphate ester hydraulic fluid, to lake trout (Salvelinus namaycush). Fry (24-days-old) were continuously exposed to 0, 2.6, 5.3, and 16 micrograms/L of Pydraul 50E for 120 days. All fish died in the 16 micrograms/L exposure, and growth was significantly less in the 5.3 micrograms/L exposure. A high incidence of light skin coloration (66%) and cataracts in one or both eyes (92%) also occurred in the 5.3 micrograms/L exposure. Bioconcentration factors for Pydraul 50E ranged from 462 to 5,500. Improved management of hydraulic fluid use is recommended to reduce environmental contamination.