Nematodes are a quantitatively significant component of the soil microfauna: the number of individuals per m2is measured in millions. They belong to the trophic level of the consumers, but they influence the processes of degradation and mineralization of organic matter, mostly due to the trophic relations which they establish with fungi and bacteria. The species that feed on fungi are represented by nematodes which are furnished with a buccal stylet and, in particular, by Tylenchida. Their role in the decomposition process of litter is essentially that of rendering the nutrients contained in fungi accessible to other organisms such as bacteria. In some cases they disseminate the spores of fungi. The nematode species that feed on bacteria have no buccal stylet. Their role consists in maintaining the demographic equilibrium in the bacterial population and in favouring the spreading and the proliferation of certain species.