Some have been recovered from samples taken at 30,000 feet. Marine forms are also abundant in Arctic and Antarctic oceans at depths of 500 or more meters. Nematodes occur in practically all soils and are common in high mountain soils 14,000 feet or more. In some agricultural soils plant-parasitic species are known to occur to depths of at least 17 feet. The distribution of many species has been influenced by human activities.
Some parasites have very broad host ranges while others have very narrow ones.
Even soil dwelling forms are living in the film of moisture that lines soil pores. It is not uncommon for several species of nematodes to be found in the same host or habitat. Marine species vary from 83 um to about 50 mm in length. The animal parasites range from about 1 mm to 6 or 7 meters in length, the largest being a nematode parasitic in whales. Plant parasites range from 0.25 mm to approximately 12 mm in length.