The genetic relationships among Mytilus galloprovincialis populations over their range in the northeastern Atlantic and the western Mediterranean were investigated using polymerase chain reaction (PCR)amplified nuclear DNA markers. We used long-range polyacrylamide gel electrophoresis for characterizing an intron-length polymorphism at the actin gene locus mac-1 in Mytilus. Sharp resolution was obtained with this technique, which revealed a high level of size polymorphism. It also allowed to discriminate between M. galloprovincialis and M. edulis. Mytilus populations sampled on the northwestern African coasts (Morocco, Western Sahara, and Mauritania) were M. galloprovincialis as formerly suggested on the basis of morphology and geographic location. A sample of the Padstow mussel reported as M. galloprovincialis according to allozyme and morphological data exhibited allele frequencies that were rather intermediate between M. galloprovincialis and M. edulis. Slight heterozygote deficiencies were possibly present within each sample as usually reported in bivalve populations. Additional PCRs using a second pair of primers showed that this could not be explained by the occurrence of null alleles that would have resulted from mis-priming during DNA amplification. Significant differentiation was observed between M. galloprovincialis from northwestern Africa and the reference M. galloprovincialis sample from the Mediterranean Sea, a result that is consistent with previous allozyme- and mitochondrial DNA-based reports of an abrupt genetic change between northeastern Atlantic / Alboran Sea and western Mediterranean M. galloprovincialis populations.