A flagellate is any member of the class Mastigophora. Flagellates are single-celled organisms that possess, at some stage of their development, one or several whip-like structures called flagella for locomotion and sensation.

They are divided into two major groups, the phytoflagellates resemble plants (in that they obtain their energy through photosynthesis), the zooflagellates resemble animals (in that they obtain their energy through feeding). Most have a single nucleus. Reproduction may be asexual (fission) or sexual.