This video shows Tetrahymena sp. (common freshwater protozoa) cells feeding on a biofilm composed of freshwater bacterium Serratia plymuthica. The bacterial cells and biofilm structures are visible due to expression by the bacteria of a red fluorescent protein. Tetrahymena sp. cells are visible as rapidly-moving clusters of intracellular feeding vacuoles packed with red-fluorescing bacterial cells. The focal plane of the video moves down with each successive frame, revealing the effect of Tetrahymena sp. feeding activity on bacterial biofilm morphology as ciliate-sized channels and holes throughout the biofilm.