We have analysed different populations of European mussels belonging to genusMytilusin order to clarify the karyotype complements of these three species (M. edulis, M. galloprovincialisandM. trossulus). We have employed different cytogenetic techniques (C-banding, fluorochrome staining and Ag-NOR staining) and we have carried out chromosome measurements by using image analysis. Results obtained allow us to confirm the diploid number of these species and to detect different karyotype composition. Furthermore, we describe cytogenetical differences between larval chromosomes and gill tissue chromosomes.