Phacus longicauda is a species of single-celled eukaryote belonging to the genus Phacus in the phylum Euglenozoa. It has an elongated spindle-shaped cell ranging from 30 to 100 micrometers in length. Covered by a rigid pellicle, it possesses two flagella: one smooth and one with fine hairs for movement. This organism contains numerous small discoid chloroplasts, giving it a green coloration for photosynthesis, and features a red or orange eyespot (stigma) for light detection. Phacus longicauda is prevalent in global freshwater habitats, contributing as a primary producer within aquatic ecosystems and adding to the biodiversity of its genus.