Rhynchopus is a protozoan flagellate in the class Diplonemea. They usually has flagella of different lengths and a single subapical opening with the flagellar pocket openings and adjacent feeding apparatus merge into one. When food is scarce, mobile flagellated cells are produced, suggesting the presence of a fully flagellated and dispersive phase in the life cycle, serving to distinguish Rhynchopus from Diplonema. Most species are free-living, others are symbionts and R. coscinodiscivorus is an intracellular parasite of diatoms.