The impact of predation by common mergansers (Mergus merganser americanus) wintering on Lake Carl Blackwell (650 ha) in Payne County, Oklahoma was investigated. Parameters measured included merganser usedays, daily food consumption, and food habits; plus the standing crops offish in the lake. There were 27,500 use-days in the winter of 1971-72 and 13,100 in the 1972-73 winter. The approximate daily food consumption was determined to be 454 g (1 pound) per merganser. Common Mergansers consumed an estimated 12.5 and 6.0 percent of the mean standing crop of fish in the winters of 1971-72 and 1972-73, respectively. Gizzard shad (Dorosoma cepedianum) comprised 84 percent of the mergansers' food by weight, and 25.6 and 12.6 percent of the standing crop of this fish was consumed in the respective winters. In 1971-72, 27.5 percent of the standing crop of white crappie (Pomoxis annularis) was consumed, as compared to 13.2 percent in 1972-73. From observations on the feeding behavior of this waterfowl it would appear that aggregations of fish are vulnerable to feeding flocks of wintering mergansers.