a genus of unicellular organisms of the class Mastigophora. The body is pear-shaped, with four (sometimes three) flagella on the anterior end; a single flagellum trails backward along the outer margin of a thin cytoplasmic membrane that connects this flagellum to the body and with it forms an undulating membrane. An elastic supporting rod, or axostyle, runs through the organism’s entire body. The nucleus is situated in the anterior part of the body.

Trichomonads reach 20 microns in length. They infest the digestive tract and sexual organs of a number of vertebrates. Three species are parasitic in man: T. vaginalis, which causes inflammation, and T. tenax and T. hominis, which inhabit the oral cavity and lumen of the small intestine, respectively; there is some doubt as to the pathogenic role of the latter two species. T. foetus is parasitic in the genital tract of cattle, causing early miscarriage, sterility, and various types of inflammation.