The spatial distribution of the subtidal crustacean assemblages of the Guadiana Riverestuary was studied previous to the building of the Alqueva Dam (the biggest dam inEurope). The differences between an estuarine and marine environment seem to be the mainreason responsible for the composition and distribution of the crustaceans along the studyzone. The Guadiana estuary has shown high number of species in comparison with othernearby estuaries and this richness seems justified by the scarce influence of pollutants(most of them have shown low or moderate values) and their hydrodynamic and granulometriccharacteristics (76 species were found, 39 in the estuarine area). A gradient ofenrichment and structuring of the assemblages was shown from the upstream to the marinezones and a spatial segregation of species was found along this estuarine environmentalgradient, e.g. the amphipod Corophium multisetosum and the isopodCyathura carinata in the upper estuary; the amphipodBathyporeia cf. pilosa and the isopodsLekanesphera levii and Saduriella losadoi in themiddle estuary; the amphipod Melita hergensis in the mouth; and a highnumber of species in the marine area. Since the Alqueva Dam will reduce the riverdischarges and may cause changes in the abiotic characteristics such as granulometry ofsediments or salinity, this study establishes a baseline against which a monitoringprogramme or follow-up studies could measure any significant effects of the dam or relatedimpacts.