This International Standard specifies a method for determining the toxicity of environmental samples on growth, fertility and reproduction of Caenorhabditis elegans. The method applies to contaminated whole fresh-water sediment, soil and waste, as well as to pore water, elutriates and aqueous extracts that were obtained from contaminated sediment, soil and waste. Juvenile organisms of the species C. elegans are exposed to the environmental sample over a period of 96 h. In the controls, the exposed test organisms are able to complete a whole life cycle within this period. A toxic effect of an environmental sample occurs if the inhibition of growth, fertility or reproduction of C. elegans in comparison to a control (aqueous control, control sediment or soil) exceeds a certain threshold value. Toxicity can by quantified by the intensity of the effect as percentage inhibition. Nematodes are the most abundant and species-rich group of metazoans in sediments and soils and play an important role in benthic and soil food webs. Nematodes are endobenthic organisms that are found at various trophic levels due to the evolution of different feeding types (bacterivorous, algal feeder, omnivorous, predators). The test organism Caenorhabditis elegans (Maupas, N2 var. Bristol) is a bacterivorous nematode that is found primarily in terrestrial soils but it also occurs in aquatic sediments of polysaprobial fresh-water systems. C. elegans is a well-studied organism and very easy to cultivate. The test is designed for measurement of the response to dissolved and particle-bound substances. It applies to the testing of sediments, soils, waste, pore water, elutriates and aqueous extracts. This document has been prepared by Technical Committee ISO/TC 147 "Water quality", the secretariat of which is held by DIN (Germany). The responsible German committee is NA 119-01-03-05-01 AK "Biotests" ("Biotests") of NA 119-01-03 AA "Wasseruntersuchung" ("Water Analysis") at the Water Practice Standards Committee (NAW). The revision of the project on the national level is funded by the Länderfinanzierungsprogramm "Wasser, Boden und Abfall" (federal funding programme "Water, soil and waste").