The order Ascaridida includes several families of parasitic roundworms with three "lips" on the anterior end. They were formerly placed in the subclass Rhabditia by some, but morphological and DNA sequence data rather unequivocally assign them to the Spiruria. The Oxyurida and Rhigonematida are occasionally placed in the Ascaridida as superfamily Oxyuroidea, but while they seem indeed to be Spiruria, they are not as close to Ascaris as such a treatment would place them.
In an alternate taxonomic treatment, the Ascaridida are ranked as an infraorder Ascaridomorpha.
The suborder Dioctophymatina is doubtfully valid; it contains a mere two families, one of which is monotypic. Most Ascaridida are placed in the suborder Ascaridina. These "worms" contain a number of important parasites of humans and domestic animals.
Important families include:
These all belong in the superfamily Ascaridoidea.