We used mark-recapture and catch-per-unit effort data to estimate abundances and distributions of three potential predators on juvenile salmonids migrating through John Day Reservoir in 1984–1986. The northern squawfishPtychocheilus oregonensiswas the most abundant predator (estimated population: 85,316), followed by smallmouth bassMicropterus dolomieu(34,954) and walleyeStizostedion vitreum(15,168). Because of uncertainty in sampling and assumptions of the mark-recapture estimator, the combined abundance of these three predators could lie between 50,000 and 500,000. We believe, however, that bias is probably negative, and that any errors should result in conservative estimates. Northern squawfish were common reservoir-wide, but large concentrations occurred immediately below McNary Dam near the head of John Day Reservoir. Walleyes were largely restricted to the upper third of the reservoir, whereas the number of smallmouth bass increased progressively downriver. As judged by abundance and distribution, northern squawfish have by far the greatest potential for predation on juvenile salmonids. We also expect predation to be unevenly distributed in time and space as a result of variations in the number and distribution of predators.