Grazing effects of ayu, Plecoglossus altivelis Temminck et Schegel, on the benthic algal assemblages were investigated in the Chikuma River, Japan. Comparison of the algal composition on boulders with and without intensively grazed patches indicated that fish grazing decreased the abundance of diatoms and prostrate filamentous cyanobacteria and caused upright filamentous cyanobacteria to predominate. Differential consumption by ayu was estimated by comparing the relative abundance of algae in the stomach contents of ayu and that in the algal assemblages within the grazed patches. The results showed that ayu consumed the prostrate filamentous cyanobacteria proportionally to their abundance, whereas they ingested diatoms and the upright filamentous cyanobacteria in a larger and lower quantity, respectively, than that expected from their abundance. Differential consumption would involve the change in the algal composition toward the predominance of upright filamentous cyanobacteria under fish grazing conditions.