Roundworm, or Nematode, a slender, cylindrical worm. Roundworms are of worldwide distribution and are found in freshwater and saltwater, in plants and animals, and in the soil. About 80,000 species are known, but zoologists estimate that nearly 1,000,000 species exist. Roundworms range in length from about 1/250 of an inch to 4 feet (0.1 mm to 1.2 m), depending on the species. They are usually colorless and are either short and spindle-shaped or long and threadlike.

Most roundworms are parasites of animals or plants. Some species are especially destructive to crops. Poisons called nematocides are used to kill these roundworms; for some crops, varieties that are resistant to roundworm infestation have been developed. Other roundworms cause such diseases as trichinosis and ascariasis in humans and other animals. Some roundworms are beneficial because they enrich the soil by breaking down dead organisms and by creating burrows through which oxygen circulates; other roundworms kill insect pests.

Roundworms make up the phylum Nematoda.