We analyzed the food habits of rainbow trout (Oncorhynchus mykiss) from Lake Oahe, South Dakota, from May through September, 1993–1994. Rainbow trout were captured in surface, suspended and bottom monofilament gill nets. Diets were analyzed using the relative importance index (RI). Composition of the diet differed among months and among size categories of rainbow trout. Greatest RI values for zooplankton prey occurred during May, July and August. Terrestrial invertebrates had the highest RI value during June and September. Overall, RI values for aquatic macroinvertebrates and rainbow smelt (Osmerus mordax) were lower than the other two prey categories. Zooplankton were the most important prey for rainbow trout < 330 mm and decreased in importance as rainbow trout increased in length. Rainbow trout between 330–459 mm fed predominantly on terrestrial invertebrates. Rainbow smelt were incorporated into the diet of rainbow trout between 201–330 mm and increased in importance as rainbow trout length increased. Rainbow smelt dominated the diet of rainbow trout > 460 mm.