ABSTRACTGrazing of fluorescent latex beads, bacteria, and various species of phytoplankton byPoterioochromonas malhamensis(Pringsheim) Peterfi (about 8.0 μm in diameter) was surveyed. The alga ingested fluorescent beads and various live or killed and nomnotile or motile organisms including bacteria, blue‐green algae, green algae, diatoms, and chrysomonads. The size range of grazed prey was from 0.1 to 6.0 μm for latex beads and from 1.0 μm (bacteria) to about 21 μm(Carteria inverse)for organisms. As many as 17 latex beads (2.0 μm) or more than 10Microcystiscells (5–6 μm) were ingested by a singleP. malhamensiscell. Following such grazing, the cell increased in volume by up to about 30‐fold. The range of cell volume of ingested prey was from 0.52 μm3(bacteria) to about 3178 μm3(Carteria inversa).This study demonstrates for the first time thatP. malhamensisis capable of grazing algae 2–3 times larger in diameter than its own cell and of grazing intact motile algae.Poterioochromonas malhamensisis an omnivorous grazer. Food vacuole formation and digestion processes were examined. The membrane that was derived from the plasma membrane and surrounded the prey disappeared sometime after ingestion. The food vacuole was then formed by successive fusion of numerous homogeneous vesicles accumulated around the prey. The prey was enclosed in a single membrane‐bound food vacuole