An order of nematodes in which the number of labia varies from a full complement of six to three or two or none. The tubular stoma may be composed of five or more sections called rhabdions. The three-part esophagus always ends in a muscular bulb that is invariably valved. The excretory tube is cuticularly lined, and paired lateral collecting tubes generally run posteriorly from the excretory cell; some taxa have anterior tubules also. Females have one or two ovaries; when only one is present, the vulva shifts posteriorly. The cells of the intestine may be uninucleate, binucleate, or tetranucleate, and the hypodermal cells may also be multinucleate.

There are eight superfamilies in the order: Rhabditoidea, Allionematoidea; Bunonematoidea, Cephaloboidea, Panagrolaimoidea, Robertioidea, Chambersiellolidea, and Elaphonematoidea. The Rhabditoidea are one of the largest nematode superfamilies and contain many important parasites of humans and domestic animals. This superfamily is distinguished by the well-developed cylindrical stoma and three-part esophagus that ends in a valved terminal bulb. In parasitic species, adult stages and some larval stages lack the valved terminal bulb. Though most species of Rhabditoidea are free-living feeders on terrestrial bacteria, others are important in the biological control of insects or as parasites of mammals. Medical parasitology