Didinium is a genus of unicellular ciliate protists with at least ten accepted species. All are free-living carnivores. Most are found in fresh and brackish water, but three marine species are known. Their diet consists largely of Paramecium, although they will also attack and consume other ciliates. Some species, such as D. gargantua, also feed on non-ciliate protists, including dinoflagellates, cryptomonads, and green algae.