[Nematodes, these roundworms that harm plants... by their roots] Plant-parasitic nematodes or round worms are major pests in agriculture. Worldwide, nematodes cause more than $100 billion in losses annually. These small worm-like organisms, each equipped with a hollow stylet, are transparent and measure from 300 to 1500 μm in length and 15 to 35 μm in diameter. Their small diameter does not allow us to see them with the naked eye but they are easily observed under a stereomicroscope. In Canada, there are 19 genera and 37 species of plant-parasitic nematodes associated with a plant disease. The majority, 16 genera and 31 species, parasitize the root system. They can cause a wide range of plant damage, from somewhat negligible to total crop loss. The severity of damage caused to the plants is related to various factors including the plant-nematode combination, environmental factors such water precipitation, soil type, and also cultural practices. When feeding, they puncture the root with their hollow stylet and thus reduce the capacity of water and nutritious elements uptake by the root system. The typical symptoms of nematode feeding are the reduction in root system, a distortion of the root structure and an increase in root diameter. Among the most prevalent pests, we must include the following endoparasitic species: the northern root-knot nematode Meloidogyne hapla Chitwood, the root lesion nematode Pratylenchus penetrans (Cobb) Filipjev & Schuurmans-Stekhoven, the stem and bulb nematode Ditylenchus dipsaci (Kühn) Filipjev, the dagger nematode Xiphinema rivesi Dalmasso, and the soybean cyst nematode Heterodera glycines Ichinohe. Even though they are found in Canada, nematodes such as the potato cyst nematode Globodera rostochiensis (Wollenweb.) Behrens and G. pallida (Stone) Behrens are quarantine organisms and must be intercepted by the Canadian Food Inspection Agency to prevent them from spreading over the territory. In Canada, the management of plant-parasitic nematodes is ensured by cultural methods (e.g. crop rotation, resistance) and chemicals.