peer reviewed ; Thirty three fish species are inhabiting the Belgian part of the River Meuse basin.The composition of these riverine fish communities and the quantitative characteristics of the specific populations are mostly determined by abiotic ecological factors linked with river morphology (slope, flow velocity), climate (water temperature, discharge regime) and water chemistry. The present paper summarizes results of observations carried out during the last twenty years. Fish communities recorded all over the river basin exhibit a high degree of diversity (in term of geographical distribution of species, number of species, particular species arrangements) that result from the influence of a few key-factors such as water temperature (allowing or not a particular fish species to spawn efficiently depending on its own thermal requirements), flow velocity (swimming capacity) and specific requirements for spawning substrate) linked with river slope and pH (no fish life possible at pH values less than 5.0). Within a river system, physical characteristics of the environment also influence the spatial and temporal variability of fish abundance. Spatial heterogeneity of depths, of flow velocities and of substratum structure results in the density and biomass of specific populations and communities being extremely variable at the levels of the morphological facies which compose a riffle-run-deep-riffle succession and of the mosaic of different habitats and microhabitats in each facies. Temporal variability of climatic factors (temperature, as in the cyprinid fish L. cephalus) or climate-dependant ones (discharge, as in the salmonid T. thymallus) determines annual fluctuations of the reproduction success which in turn govern the population dynamic and productivity. Finally, the paper points out some negative ecological consequences (extinction of fish species, decrease of fishery resources)) caused by man activities changing the water chemistry, water temperature (thermal waste + earth climate change including ...