Trimastix is a genus of small, bacterivorous, free-living zooflagellates from anoxic and microaerophilic marine and freshwater habitats. The species, some of which are in the literature as species of Tetramitus, are known from marshes, wastewaters, and other similar places. Their distributions are not well known due to the small number of records.

The genus name is composed from the Greek words "three" and "thread", reflecting the belief of W. Saville Kent, who described the type species in the 1880s, that the cells have three flagella. Trimastix cells actually have four flagella, but the name is retained because it is the first one that was used to identify and describe them.

Trimastix species lack mitochondria, but have small, membrane-bound organelles that may be hydrogenosomes, hydrogen-excreting organelles that are believed to be descended from mitochondria.

The Trimastix species most closely resemble the free-living jakobid flagellates, most of which have mitochondria, and the endocommensal/parasitic retortamonads, which lack mitochondria and hydrogenosomes. Studies on these organisms may provide important information on the origin and early evolution of mitochondria in eukaryotic cells.

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