SynopsisData on spatial variation of sculpin density, growth and fecundity support the hypothesis that populations of stream fish are structured by changes in risk of predation and prey availability along a gradient in stream size. Cot&s bairdi in warm streams and C. cognate in cold streams exhibit similar patterns. Sculpins in large streams have faster individual growth rates and higher fecundities than those in small streams, but occur at lower density. The patterns appear to be persistent and suggest that predation reduces sculpin density in larger streams. Competitive release, variations in prey productivity, and local factors probably contribute to the variation in sculpin growth.