The nematodes like insects are a highly diversified group of animals and occur in all possible kinds of climatic conditions and habitats. For the existence and survival of nematodes, free-living as well as parasitic, either water or liquid medium is necessary. The studies so far carried out in India show that several groups of nematodes are present in the paddy fields including those which are specific parasites of the roots, stem and leaves of rice plants. The predaceous nematodes, particularly those belonging to the Order Mononchida feed voraciously on both, free-living and plant-parasitic nematodes and hence are important tools in the biological control of phytophagous nematodes. The plant-parasitic nematodes that are associated with paddy belong to several groups with different kinds, characteristics, feeding habits, behaviour and pathogenicity. A number of species of nematodes belonging to Hirschmanniella, Pratylenchus and Hoplolaimus enter the root system and feed as migratory endoparasites but these may also be recognised as migratory ectoparasites.