We examined the relationships between the distribution of dominant herbivorous insect grazers (Glossosoma larvae), environmental factors (current velocity, water depth, periphyton biomass), and grazer–periphyton interactions at multiple spatial scales (microhabitat, riffle, reach) in a stream. We used multiple regression models to explain densities of Glossosoma larvae at each spatial scale in terms of the environmental factors. All r²-values were significantly higher at the riffle than at the microhabitat or reach scales. Thus, the riffle scale provided better predictions of Glossosoma larval density than did the microhabitat (smaller) and reach (larger) scales. The r²-values of exponential regressions between grazer densities and periphyton biomass were lower at the microhabitat than at the riffle or reach scales. These results indicate that the patterns of relationships between the insect grazers and periphyton were detected more clearly at larger than at smaller scales.