We collected gut contents from northern squawfish Ptychocheilus oregonensis captured in the Clearwater River, Idaho, 0–6 km from its confluence with the Snake River, following the release of 1.1 million yearling chinook salmon Oncorhynchus tshawytscha from the Dworshak National Fish Hatchery. Before the hatchery release, northern squawfish gut contents (by weight) in the study area were 38% crayfish Pacifastacus spp., 26% insects, 19% nonsalmonid fish, and 16% wheat kernels Triticum spp. Juvenile salmonids constituted 54% of gut contents about 24 h after the hatchery release, 78% after 5 d, and 86% after 7 d. The mean number of salmonids per gut (1.2) after release was higher than typically seen in guts from northern squawfish collected in mid-reservoir areas away from hydroelectric dams on the Snake and Columbia rivers. Length-frequency distributions of juvenile salmonids eaten and those captured in a scoop trap 4 km upstream of the study area indicated that northern squawfish were selectively feeding on the smaller individuals. We attribute the high rates of predation in the study area to the artificially high density of juvenile salmonids resulting from the hatchery release and to the physical characteristics of the study area in which the river changed from free flowing to impounded. Our results suggest that northern squawfish can quickly exploit hatchery releases of juvenile salmonids away from release sites in the Columbia River basin.