Increased mortality of rainbow trout (Salmo gairdneri) has been observed during the fall and early winter at Greers Ferry National Fish Hatchery since it began operation in 1966. Located in east central Arkansas, the hatchery uses aerated hypolimnetic water from Greers Ferry Reservoir. Positive correlation between trout mortality and manganese concentration in the hatchery was obtained during the period from 8 November through 1 December 1967. After 1 December, the manganese concentration continued to increase but mortality dropped. Immediately before complete mixing of the reservoir, a second peak in mortality was observed. The presence of oxidized forms of manganese in the reservoir at the interface between the anoxic and oxygenated zones is proposed as a possible explanation for the observed mortality.