In the Columbia River, we found the diet of subyearling chinook salmonOncorhynchus tshawytschato be predominately caddisflies (Trichoptera; 64% by weight) in riverine nursery habitats, but mostlyDaphniaspp. and terrestrial insects in reservoir habitats. A preference analysis indicated that subyearlings did not eat food items in proportion to food availability; they preferred small zooplankters least and terrestrial insects most. Rank in preference of food items was correlated positively with rank in size of available food items, but not with rank in number in the diet. Only rank in number of prey items available in the habitat was consistently correlated with rank in number in the diet. The shift in diet toDaphniaspp. and terrestrial insects in the reservoir was due primarily to the high availability of those food items and the low availability of others. The switch to food items abundant in reservoirs enabled subyearling chinook salmon to use the reservoirs as nursery areas. However, the use ofDaphniaspp. may entail a higher foraging cost per energy unit gained because of the small size of the prey.