This part of ISO 23611 specifies a method for sampling and handling free-living nematodes from terrestrial field soils as a prerequisite for using them as bio-indicators (e.g. to assess the quality of a soil as a habitat for organisms). This part of ISO 23611 applies to all terrestrial biotopes in which nematodes occur. The sampling design of field studies in general is specified in ISO 10381-1. This part of ISO 23611 is not applicable to aquatic nematodes because these nematodes do not pass through the filter. Methods for some other soil organism groups such as earthworms, enchytraeids or collembolans are covered in other parts of ISO 23611. The nematodes that are characterized by the proposed procedure are all the free-living forms of nematodes found in soil. They include non-plant-feeding nematodes as well as ectoparasitic plant-feeding nematodes and free-living stage of endoparasitic nematodes. The quantification of obligate plant-feeding nematodes in roots requires specific methods. NOTE Basic information on the ecology of nematodes and their use as bio-indicators can be found in the bibliography.This part of ISO 23611 does not cover the pedological characterization of the site which is highly recommendable when sampling soil invertebrates. ISO 10390, ISO 10694, ISO 11272, ISO 11274, ISO 11277, ISO 11461 and ISO 11465 are more suitable for measuring pH, particle size distribution, C/N ratio, organic carbon content and water-holding capacity.