The genetic composition of wild and cultured mussel populations from the Oosterschelde, The Netherlands, is compared to representative samples of mussel shipments from culture sites in Ireland and Great Britain. A total of 2272 mussels from 27 samples were assayed for the nuclear DNA markers Me 15116, EFbis and M7. The three markers showed similar patterns of taxonomic consistency across the mussel samples. However, some variation between markers was observed. While, all samples were almost "pure" Mytilus edulis, nevertheless Dutch wild populations Were more admixed with M. galloprovincialis and M. trossulus alleles than those from eastern Ireland, Wales and southern England. The mixed allele genotypes (hybrids) indicate the pre-existing occurrence of non-indigenous Mytilus taxa or natural introgression into Oosterschelde. Our results demonstrated that at the genetic level. there is no immediate threat from the introduction of nonindigenous Mytilus taxa through import from the studied sites in Ireland and Great Britain into the Oosterschelde. However, it is possible that with the import of mussels, M. galloprovincialis will be translocated, especially in the face of expected climate changes in the future.