Phylogenetic tree based on protein-coding genes sequences of nine species of Polyopisthocotylea, with the Monopisthocotylea Cichlidogyrus casuarinus as outgroup. The tree has two large clades. The first one contains only the Diplozoidae. The second includes members of the Mazocraeidea and is subdivided into two smaller subclades, with one containing the Microcotylidae and the other containing the Chauhaneidae, Diclidophoridae and Hexostomatidae (H. thynni). Hexostoma thynni is not the sister-group to the other Mazocraeidea.