Tritrichomonas muris is a flagellated protist isolated from the cecum of wild mice in the Czech Republic. This commensal protist has been shown previously to alter immune phenotypes in laboratory mice. Other trichomonads, previously referred to as Tritrichomonas musculis and Tritrichomonas rainier , also naturally colonize laboratory mice and cause immune alterations. This report formally describes two new trichomonads, Tritrichomonas musculus n. sp., and Tritrichomonas casperi n. sp., at the ultrastructural and molecular level. These two protists were isolated from laboratory mice, and were differentiated by their size and the structure of their undulating membrane and posterior flagellum. Analysis at the 18S rRNA and trans- ITS genetic loci supported their designation as distinct species, related to T. muris . To further assess the true extent of parabasalid diversity infecting laboratory mice, 135 mice were screened at the NIH using pan-parabasalid primers that amplify the trans- ITS region. Forty-four percent of mice were positive for parabasalids, encompassing a total of 8 distinct sequence types. Tritrichomonas casperi and Trichomitus- like protists were dominant. T. musculus and T. rainier were also detected, but T. muris was not. Our work establishes a previously underappreciated diversity of commensal trichomonad protists that naturally colonize the enteric cavity of laboratory mice.