Protozoan

Introduction
Features of protozoans
Natural history
- Size range and diversity of structure
  - Flagellated protozoans
  - Amoebae and pseudopodia
  - Ciliated protozoans
  - Parasitic protozoans
- Distribution and abundance
- Ecological and industrial importance of protozoans
- Protozoans and disease
Form and function
- The protozoan cell
- Characteristics of locomotion
  - Flagellar propulsion
  - Cilium structure and beat
  - Amoeboid movement
- Respiration and other energy-generating pathways
  - Aerobic protozoans
  - Anaerobic protozoans
    - Hydrogenosomes
    - Mitosomes and glycosomes
- Carbon acquisition and nutrition
  - Mechanisms of food ingestion
  - Food vacuoles
  - Food selection
  - Mixotrophy
    - Photosynthesis and plastid acquisition
    - Symbiotic mixotrophy
    - Mixotrophy in planktonic protozoans
- Reproduction and life cycles
  - Mechanisms of asexual reproduction
  - Mechanisms of sexual reproduction
    - Conjugation in ciliates
    - Autogamy and modified conjugation
  - Parasitic protozoan life cycles
- Adaptations
Evolution and paleontology
Classification
- General principles
- Diagnostic features
- Annotated classification

References & Edit History Quick Facts & Related Topics

Images & Videos

representative protozoans

Discover how individual cilia employ viscous drag to coordinate power and recovery strokes for locomotion

glass model of Volvox

Dead fish scattered along the sandy shore of Tampa Bay, Florida, with reddish and greenish water bands near the coast caused by a red tide dinoflagellate bloom.

oligotrich; Halteria grandinella

A parasite in a parasite

blackwater fever

Analyze the structure and motion of cilia and flagella

At a Glance

default image

protozoan summary

Related Questions

Protozoan

microorganism

Also known as: Protozoa

Written by

Johanna E.M. Laybourn-ParryAll

Fact-checked by

Britannica Editors

Last updated

Nov. 12, 2026 •History

Protozoan, organism, usually single-celled and heterotrophic (using organic carbon as a source of energy), belonging to any of the major lineages of protists and, like most protists, typically microscopic. All protozoans are eukaryotes and therefore possess a “true,” or membrane-bound, nucleus. They also are nonfilamentous (in contrast to organisms such as molds, a group of fungi, which have filaments called hyphae) and are confined to moist or aquatic habitats, being ubiquitous in such environments worldwide,...

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