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Most species within this group are parasites or endosymbionts within vertebrates and invertebrates. Several species are important parasites of domestic animals (e.g., Histomonas meleagridis, Tritrichomonas foetus) and humans (e.g., Trichomonas vaginalis). The trophozoites are often ovoid or pear‐like in shape: the anterior is usually rounded and the posterior pointed, although amoeboid forms occur in some species. The number of flagellae varies between species, but there are often 4–6 emerging at their anterior apex. In addition, one flagellum usually curves backwards so that it runs along the cell wall to form an undulating membrane – this flagellum is therefore described as ‘recurrent’. Most species do not form cysts. They have a single nucleus and internally there is a prominent median tube‐like organelle called the axostyle.

Trichomonads have genomes ranging from 86 to 177 Mb. This is unusually large for protozoa: by comparison, the genome of Plasmodium falciparum is ~23 Mb, whilst that of Trypanosoma cruzi is ~34 Mb. The large size is largely a consequence of extensive gene duplication, and this may contribute to their success as parasites and facilitated their ability to infect a variety of host species (Barratt et al. 2016). Gene duplication also occurs in other parasitic protozoa although not usually to the same extent as in the trichomonads. Gene duplication is not, however, an invariable feature of a parasitic lifestyle and some parasitic protozoa have unusually small genomes. For example, in the apicomplexan Babesia bovis, it is 8.2 Mb, whilst the microsporidian Encephalitozoon intestinalis has a genome of only 2.3 Mb.