Summary The monophyly of the trichostomes was tested using new 18S rRNA gene sequences from the rumen ciliates, Dasytricha ruminantium (1,638 bp) and Isotricha intestinalis (1,639 bp), and the haptorian, Didinium nasutum (1,643 bp). In all parsimony and distance trees, Dasytricha and Isotricha always paired together as the vestibuliferids (Order Vestibuliferida). The vestibuliferids were united with a clade consisting of the entodiniomorphid rumen ciliates (Order Entodiniomorphida) to form the trichostomes (Subclass Trichostomatia). In the alternative maximum parsimony tree and distance-matrix trees, Didinium grouped with Homalozoon and Spathidium to form a clade (Order Haptorida) that was the sister group to the pleurostome, Loxophyllum (Order Pleurostomatida). Together, these latter four genera represent haptorian ciliates (Subclass Haptoria), which along with their sister group, the trichostomes, comprise the litostomes (Class Litostomatea).