The Phylum Dinoflagellata is in the Kingdom Alvoelata, part of the former Kingdom Protista, now broken up into many kingdoms. Dinoflagellates are characterized by their flagella located in grooves in cellulose plates surrounding the cell. They also have membrane bound sacs known as alveoli in the plasma membrane. It is unclear what the function of these alveoli are, but perhaps they help stabilize the surface of the cell, as well as regulate water and ion content.

A component of phytoplankton, dinoflagellates are found mostly at water surfaces. They contain chlorophyll a and c, as well as other carotenoid and xanthophyll pigments. When dinoflagellates bloom, or explode in population size, they actually change the water to a reddish brown color due to these pigments. This phenomenon causes red tides, which can have a deadly affect to both marine animals and humans. Recently, the outbreak of red tide has increased, and one possible reason may be water pollution with fertilizers, which contain nitrates and phospates that the dinoflagellates take up.

Dinoflagellates can be either heterotrophic, autotrophic, parasitic, or symbiotic. Many are autotrophic, photosynthetic protists and primary producers. The heterotrophs, representing half of all the species, are usually the more dangerous ones, as they are carnivorous and produce the deadly toxins to stun marine life such as fish. Occasionally, these heterotrophs can extract chloroplasts when they ingest photosynthetic protists, becoming temporarily autotrophic. Some dinoflagellates are both autotrophic and heterotrophic. Those that live as mutualistic symbionts are called zooxanthellae. They are found in a range of marine invertebrates, including coral, sponges, and jellyfish. When they are living with cnidarians, they serve as the main souce of photosynthesis for the coral reef community. And finally, the dinoflagellates that lack chloroplasts become parasitic within marine animals.

Asexual reproduction is usually the method employed by dinoflagellates to create its progeny. Simple mitosis and cell division occur to produce identical daughter cells. Rarely does sexual reproduction occur, causing gametes to fuse to form a motile planozygote.