Abstract In a consideration of nematode and plant inter-relationships, five nematode trophic groups must be considered: microbivores, fungivores, parasites of higher plants, predators, and omnivores. In uncultivated areas, such as deciduous forests, many species of nematodes are likely to be present (often 100 to 200 species in temperate areas) with the trophic structure stable and dependent on topographic, edaphic, vegetative and climatic factors that characterize the habitat. When formerly uncultivated lands are placed in cultivation, some species build up, and other members of the nematode community decline and even disappear. Soils of perennial crops may have a nematode species diversity comparable to soils of undisturbed areas. Soils planted to annual crops, however, usually have a low species diversity although nematode densities may be high during certain seasons of the year. Nematode community structure depends on a number of factors including soil type and crop grown, and may shift from year to year depending on cropping history. In the tropics monocultures and their subsequent effect on nematode communities are an even greater departure from the natural state than they are in temperate zones. Primary control strategies for nematodes have included the use of nematicides and resistant varieties. Also used are crop rotation, summer or winter fallow, trap cropping, and other techniques. New strategies must be developed by which all biological and physical aspects of the feedback system can be managed.