Grazing on the harmful, bloom-forming cyanobacterium Microcystis aeruginosa by the heterotrophic flagellate Collodictyon triciliatum was examined in an experimental pond with Microcystis bloom in the summer of 2010. The grazing rates were estimated from digestion rate and food vacuole content of C. triciliatum. M. aeruginosa had one blooming with the maximum density of 1.1 x 10⁵ cells ml^-1. Cell density of C. triciliatum fluctuated between 0 to 290 cells ml^-1. The number of M. aeruginosa cells ingested into food vacuoles of C. triciliatum ranged between 0.4 and 10.8 cells flagellate^-1. We finally estimated grazing rates on M. aeruginosa pray by C. triciliatum as 0.2–6.9 cells flagellate^-1 h^-1, and its grazing impact was 0.0–25.3% standing stock day^-1. The functional response of C. triciliatum on M.aeruginosa prey was also examined with Michaelis-Menten model in experimental systems where the prey concentration varied from 6.4×10³ to 2.1×10⁶ cells ml^-1. Our application of the model was successful (r² = 0.873, p < 0.001). The maximum grazing rate was estimated as 6.2 prey cells grazer^-1 h^-1 , and half-saturation constant as 1.2×10⁵ cells ml^-1 . We present evidence that C. triciliatum grazing explained the remarkable decrease in M. aeruginosa cell density in the pond.