The parabasalids are group of flagellate protozoa, most of which are symbiotic in animals. These include a variety of forms found in the guts of termites and cockroaches, many of which have symbiotic bacteria that help them digest wood. Some other species are human pathogens.

The flagella are arranged in one or more clusters near the anterior of the cell. Their basal bodies are linked to parabasal fibers that attach to prominent Golgi complexes, distinctive to the group. Usually they also give rise to a sheet of cross-like microtubules that runs down the center of the cell and in some cases projects past the end. This is called the axostyle, but is different in structure from the axostyles of oxymonads.

Parabasalids are anaerobic, and lack mitochondria, but this is now known to be a result of secondary loss, and they contain small hydrogenosomes which apparently developed from reduced mitochondria. Similar relics have been found in other amitochondriate flagellates, and the parabasalids are probably related to them, forming a group called the metamonads. They lack the feeding grooves found in most others, but this is probably a secondary loss as well.

There are two groups of parabasalids. The trichomonads have one group of 4-6 flagella, one of which is attached to the side of the body and often forms an undulating membrane. Many are found in vertebrate hosts, including Trichomonas vaginalis, which causes a sexually transmitted disease. A few are found in insects, or are free-living. The hypermastigids have a large number of flagellar clusters, and are found exclusively in the guts of insects.