ABSTRACT Flagellates display all of the basic trophic strategies seen among both autotrophic and heterotrophic eukaryotes. Photoautotrophic flagellates employ a diversity of different pigment combinations in energy capture for photosynthesis. Their wide size range enables them to exploit habitats that are rich or poor in mineral nutrients. Autotrophic members of several groups have established successful and important symbiotic relationships with other protists and higher animals. Many of the studied examples of photosynthetic flagellates are auxotrophs, depending on an external supply of one or more vitamins.