Although gammarid species are promising bioindicators of metal bioavailability in freshwaters, interspecies variability is still rarely assessed in situ. Besides, ambient environmental conditions inherent to the species' occurrence are likely to affect metal bioavailability, hence hampering the interpretation of field bioaccumulation data. The study aimed to assess the relevance of using common gammarid species for biomonitoring water quality at the scale of a river basin. Gammarids were collected at 20 sites of the Seine basin during three consecutive years to establish geographical distribution patterns of the species and their metallic contamination levels. The sites were characterised in terms of physiography, physicochemistry, hydrothermal regime and land use to identify ecological factors explaining the absence or presence of species. Bioaccumulation abilities of Cd, Co, Cu, Mn, Pb and Zn were compared between species as a function of the exposure levels and environmental characteristics of their occurrence. Statistical analyses indicated that the occurrence of Gammarus fossarum and Gammarus pulex, as native species of the Seine basin, is governed by a combination of entangled environmental factors. By contrast, Echinogammarus berilloni as a naturalized species tolerated larger ranges of environmental conditions and related-land use pressures. At some sites, the absence of gammarid species was associated with unfavourable conditions, e.g. eutrophication, urbanisation. Unlike Pb, bioaccumulation abilities appeared to be comparable among species for Cd, Cu, Mn and Zn, and only between both native species for Co, when local physicochemical conditions were considered. For further, the generic responsiveness to exposure levels supported that metal contents in gammarids provide reliable information on metal bioavailability whatever physicochemistry of freshwaters and ecological zonation of these common species of the Seine basin.