Protozoa exist throughout aqueous environments and soil, occupying a range of trophic (nutritional) levels. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi.

Protozoa play a role as both herbivores and consumers in the decomposer link of the food chain.

Protozoa also play a vital role in controlling bacteria populations and biomass and they may absorb food via their cell membranes, some; for example, amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food.

All protozoa digest their food in stomach-like compartments called vacuoles.