Protozoan, a one-celled organism. A protozoan is a eukaryote, an organism characterized by having its hereditary material enclosed in a nucleus bounded by a membrane. Most protozoans are microscopic, ranging in size from about 0.001 to 0.01 mm in length, but some, including certain amoebas, are large enough to be seen with the unaided eye.

Free-living protozoans feed mainly on microscopic organisms—bacteria, yeasts, algae, and other protozoans. A few species contain chlorophyll and are able to make their own food by photosynthesis. Many protozoans eat dead matter, and thus are useful in disposing of organic wastes.

All nonparasitic protozoans are basic organisms in the animal food chain and pyramid of numbers. These terms refer to the feeding and population relationships of large animals to successively smaller, more numerous animals. For example, a person may eat several fish that have fed on hundreds of shrimp that had consumed thousands of protozoans. Thus, protozoans are of indirect but fundamental importance to humans.


The body of a protozoan resembles, in a general way, a single cell of a metazoan, or many-celled animal. However, the protozoan body often has special structures called organelles ("little organs") not found in metazoan cells. Organelles perform functions that are carried out by tissues and organs in higher forms of life. For example, many protozoans possess organelles called cilia ("hairs") or flagella ("whips")—tiny projections used in swimming. Other protozoans move about by extending a part of the body to form a pseudopodium ("false foot"), into which the rest of the body then flows. Many protozoans have more than one nucleus. (Virtually all metazoan cells have single nuclei.)

Unlike a metazoan cell, the protozoan body may include a delicate, geometrically patterned skeleton of silica, lime, or other hard substance. Some protozoans have trichocysts—fine, threadlike structures with barbed tips that serve to defend the protozoan, anchor its body, or paralyze its prey. Many species of protozoans form colonies that superficially resemble branching plants or sac-like metazoans.

Protozoans commonly take in food and water, and give off wastes, by simple osmosis, the exchange of soluble materials through the body wall. (Osmosis also is typical of metazoan cells.) However, many protozoans have organelles for capturing food. Some have a well-defined gullet, or mouthlike groove. Others use pseudopodia to surround their food, or use cilia to create water currents that draw the food nearer. Inside the body, digestion may take place in organelles called food vacuoles. Organelles called contractile vacuoles forcibly expel excess water from the cell.

Protozoans commonly reproduce by binary fission (division of the body into two equal parts). Some reproduce by budding (division of the body into two or more unequal parts). Under certain conditions, fission or budding may be preceded by conjugation (exchange of nuclear material during contact between two individuals of the same species). Parasitic protozoans usually have a complicated life cycle that includes the production of gametes, or sexual cells, and spores, or asexual cells. A spore may possess a cyst—a covering that protects the spore in unfavorable surroundings or during transmission from place to place. Some protozoans live for years in the encysted state.

Protozoans belong to the kingdom Protista. Some biologists place them in the phylum Protozoa and divide them into the following classes: Mastigophora (or Zoomastigina), flagellate forms; Sarrodina (or Rhizopoda), pseudopodal forms; Ciliata (or Ciliophora), ciliated forms, Sporozoa (or Apicomplexa), parasitic forms with spores that infect other organisms; and Cnidospora (orCnidosporidia), parasitic forms with spores that do not infect other organisms. Other biologists do not recognize a phylum Protozoa and consider the various types to make up phyla rather than classes; still others separate protozoans into additional classes or phyla.