Diet, foraging behavior, growth, sexual maturity, and spawning site selection were studied in a wild population of exotic rainbow trout Oncorhynchus mykiss in the spring-fed Horonai stream in Hokkaido, Japan. During spring and summer, the rainbow trout ate primarily aquatic and terrestrial invertebrates, but eggs and larvae of river sculpin Cottus nozawae constituted 10% of the diet by number in spring. Rainbow trout typically held focal points in the stream flow and intercepted food items in the drift. Sexually mature individuals caught in late January were ages 1-5⁺. Males ranged 11.0-36.5cm and females 16.8-33.1cm in fork length. Females constructed spawning redds in calm riffles with fine gravel substrate. Our re-sults suggest that introductions of rainbow trout may have detrimental effects on Japanese stream fishes and other aquatic biota through interspecific competition for food and space and/or predation.