Mixotricha is a large polymastigote flagellate from the gut of the termite Mastotermes darwiniensis. Its behaviour and fine structure are described in detail. A study of the method of locomotion, using cinematography, has shown that the four flagella are used not to propel the organism but to steer it; movements are brought about by the co-ordinated undulations of many thousands of spirochaetes which cover most of the body surface. The spirochaetes are attached to small brackets, of complex internal structure, which arise in rows from the cell surface. In addition to one or more spirochaetes, each bracket is also associated with an extracellular bacterium. The fine structures of both spirochaetes and bacteria are described. A network of fibrous strands lies immediately below the cell surface, each of its meshes being associated with one of the over-lying brackets. The cytoplasm contains great numbers of vacuoles, as well as dictyosomes and rough-surfaced membranes. The membranes forming these structures, as well as the membranes of the nuclear envelope and the plasma membrane, are all triple-layered but differ in dimensions and appearance. The rough-surfaced membranes are always associated with intracellular bacteria. The taxonomic position of Mixotricha is discussed in the light of the findings on its structure and behaviour.