Description of Amastigomonas mutabilis: Cells are elliptical, 11 to 16 microns long, dorso-ventrally flattened and flexible. There is a flexible sleeve around the base of the anterior flagellum. The anterior flagellum is about 0.5 times the length of the cell and is the same thickness as the posterior flagellum. The recurrent posterior flagellum is slightly longer than the cell and trails under the body, to which it attaches loosely in a slight groove. The nucleus is situated subapically near the right margin of the cell. Some cells have granules along side the recurrent flagellum.