The ascarids are important parasites of humans, domestic and wild animals. Some species have a severe economic impact.
Life cycles can be direct, or indirect with one or more intermediate or paratenic hosts. These are parasites as adults in fish, amphibians, reptiles birds and mammals, and larval stages can infect fish or other vertebrates.
Ascarids are large nematodes, 5-30 cm long, and the body is usually stout. A characteristic feature is the presence of three, large lips around the mouth. The esophagus is simple, but there may be one or more appendices arising from near the esophagus-intestine junction. The male does not possess a copulatory bursa, but various papillae and suckers may be present at the posterior end of the body.