The extent of pollution of a river is often ascertained by examining the bottom meso- and macro-fauna, and standardized testing schedules have been proposed to express the « health condition)} of the water in comparative quantitive terms (Woodiwiss, 1964; Tuffery and Verneaux, 1968; Sladecek, 1973). However, the molluscs, annelids and insect larvae which are the animal group used, are not always sufficiently abundant to allow statistical analysis, and insect populations fluctuate with seasonal conditions so that a satisfactory appraisal can therefore be established only by taking many samples and at several times during the year. Moreover, these animal groups, with the exception of oligochaetes (Brinkhurst, 1962), are virtually absent at high levels of pollution. The present paper proposes the use of nematodes as indicators of river pollution. They are universally present in the river bed, regardless of whether it is stony, muddy or sandy; populations are not subject to seasonal fluctuations to any extent; and some groups are present even under the most extreme conditions of pollution.
