Mansonella has been established as a valid genus, and Tetrapetalonema synonymized with it. Comparative morphological studies have resulted in the restructuring of the Mansonella group. The genus is characterized by the following features: cuticular pre-esophageal ring absent; esophagus slender, poorly differentiated; female tail with four terminal papillae; caudal papillae in male typically clustered around cloaca; and, microfilariae without sheath. Twenty-four species are assigned to the genus Mansonella. Five subgenera are proposed to accommodate these species. The subgeneric groups are distinguished principally on morphological criteria, although host range and geographical distribution are considered. The following systematic classification is proposed: Mansonella (Mansonella) subgen. n., to accommodate those species which have the following morphological features: cephalic extremity with dorsoventral orientation; male tail with tip flattened dorsoventrally; vulva in midesophageal region; and microfilariae without nuclei extending to tip of tail. Type species is M. (M.) ozzardi. Mansonella (Tupainema) subgen. n., to accommodate the single species M. dunni with the following characters: cephalic extremity with lateral orientation and body constricted to form cephalic extremity in shape of hemisphere narrower than body behind it; tip of male tail not flattened dorsoventrally; vulva at or posterior to base of esophagus; and, microfilariae without nuclei to tip of tail. Mansonella (Esslingeria) comb. n., to accommodate those species with the following characters: cephalic extremity with lateral orientation; male tail flattened dorsoventrally at tip or not; vulva in midesophageal region; and, microfilariae with nuclei to tip of tail. Type species is M. (E.) perstans. Mansonella (Tetrapetalonema) comb. n., to accommodate those species with the following characters: cephalic extremity with dorsoventral orientation or with body constricted to form cephalic extremity in shape of hemisphere narrower than body behind it; male tail not flattened at tip; vulva in midesophageal region; and, microfilariae with nuclei to tip of tail. Type species is M. (T.) marmosetae. Mansonella (Sandnema) comb. n., to accommodate those species with the following characters: cephalic extremity rounded, not oriented along lateral or median axis; tip of male tail not flattened; caudal papillae on male tail not clustered around cloaca; vulva near base of esophagus; and, microfilariae with nuclei to tip of tail. Type species is M. (S.) digitata.