Amastigomonas (also called Thecamonas (Larsen and Patterson, 1990)) is a very widespread and common genus. The genus has been reported from around the world, most records are from marine habitats, but it has also been reported from soils and freshwaters. The organism glides on the substrate with its anterior flagellum moving slightly. The genus eats bacteria, and food is ingested using pseudopodia that are produced by the ventral surface.

Twelve nominal species have been assigned to this genus, but a number of these are barely distinguishable and have been rendered into synonymy. The most detailed survey of the genus is that of Molina and Nerad (1991), and the most recent comments are those of Lee and Patterson (2000).

The species in this genus have the characters typical of all apusemonads. No generic synapomorphies have been found to date.

Biflagellate gliding protists, flagella insert subapically and to one side, dorsal surface of the body is covered with a thin organic theca, and the ventral surface produces pseudopodia. Anterior flagellum is enclosed basally or completely by the theca. Posterior flagellum trails under the body, both flagella may be very difficult to see. From marine and freshwater sites. Several species; Type species A. debruynei de Saedeleer 1931.

• Amastigomonas
  • Amastigomonas bermudensis
  • Amastigomonas debruynei
  • Amastigomonas filosa
  • Amastigomonas mutabilis
  • Amastigomonas terricola
  • Amastigomonas klosteria
  • Amastigomonas gigantea
  • Amastigomonas griebensis
  • Amastigomonas marina
  • Amastigomonas muscula

Sister group relationships within the genus have not been explored. It may be that the genus is paraphyletic, and that Apusomonas cannot even serve as an outgroup.