This paper documents the seasonal and diel movements of seaward-migrating juvenilePacific salmonids (Oncorhynchus spp.) passing John Day Dam on the Columbia River. Duringthe 1987-1989 and 19911993 seasons, the 10, 50, and 90% passage dates each fell within a 2-week period for yearling Chinook salmon (O. tshawytscha), steelhead (O. mykiss), coho salmon(O. kisutch), and sockeye salmon (O. nerka). For subyearling Chinook salmon, the 10 and 90% passage dates varied by as much as 1 month, although the median passage dates occurredwithin a 10-d period. The percentage of fish that passed at night (2200-0600 hours) averaged80.7% for yearling Chinook salmon, 75.7% for subyearling Chinook salmon, 77.9% for steelhead,88.6% for coho salmon, and 78.3% for sockeye salmon. Information concerning the hourly andseasonal movements of juvenile salmonids through hydroelectric dams are essential in makingwise fisheries resource and river flow management decisions.