The first internal transcribed spacer (ITS-1) of ribosomal RNA of 5 individuals of unknown pisidiid clams collected over several years in the Rhone river basin was extracted to specify their taxonomic position. The phylogenetic tree obtained from the comparison of these sequences to those of 19 species available in the NCBI genetic database is in good agreement with the phylogenetic structure of Sphaeriinae and revealed that the unknown bivalves belong to the North American species Euglesa compressa. This invasive mollusk has now colonized the major French river basins and could be responsible for the major decline of E. supina in the areas where it has settled. The main features of E. compressa are also presented.