Trepomonads (diplomonads) are unicellular flagellates. Most species inhabit in guts of animals, but some species of Trepomonas and Hexamita are free-living in anaerobic environment such as sediments. Some diplomonads (e.g. Giardia intestinalis) infect and damage human and domestic animals.

The most species possess two karyomastigont systems in each of which four flagella (basal bodies) are associated with a nucleus. Some species (e.g. Enteromonas, Trimitus) possess only a single karyomastigont system. Traditionally, the later was considered to be primitive to produce the former. However, recent molecular phylogenetic stidies suggest that the later is originated from the former by a reduction of the karyomastigont systems. Mitochondrion transformes to mitosome to lose mitochondrial genome, cristae and the ability of aerobic respiration. There is no typical Golgi body. Some species such as Trepomonas and Hexamita possess cytostome and are phagotrophic but others (e.g. Giardia) have no cytostome and are osmotrophic. In Hexamita and Spironucleus codon usages of TAA or TAG are glutamine, but those are stop codons in Giardia as in other eukaryotes (Keeling & Doolittle 1997. Mol. Biol. Evol. 14: 895-901).