The present study is part of a convention passed with the Région Wallonne. It aims at completing the examination of the hydrographic network in Wallonia under the angle of diatoms, applying the Diatom Biological Index (IBD, Lenoir & Coste 1996) Siliceous algae like diatoms, that are part of the periphyton, can be used to evaluate the biological quality of running waters. Given their specific sensitivity to pollution, applying the IBD makes it possible to obtain an evaluation of the degree of organic pollution and eutrophication sustained by all the streams and rivers under scrutiny, the measure of which is relatively independant on their degree of physical alteration. The examination of biological quality based on vegetal indicators therefore contributes to an approach of water quality based on various living communities, representative of the environment. The level of performance of the IBD, accepted as norm in France since June 2000, is being tested comparatively to physico-chemical and biological data obtained from the network of measures established in Wallonia, and with respect to the specific polluosensitivity index (IPS) considered as a reference method (Coste, in CEMAGREF 1982). The number of stations where these bioindicators have been sampled during three field campaigns (spring 1999, autumn 1999 and spring 2000) amounts to 143, 114 of which belong to the Meuse catchment and 29 to the neighboring Escaut (Schelde) basin. In a first step, we focused on the variations in water quality observed with time over a period of ~20 years, using data sets dating back from the period 1978-1980, during an inventory of water quality in the Meuse catchment (Descy et al 1982). All samples are examined under the microscope and the results of floristic analyses on diatoms are treated by a software package equipped with the routines necessary for the calculation of the european diatom-based indexes (OMNIDIA 3).