Biometrical analyses have been carried out on 26 population samples of Carcinus from the Pacific (1), Atlantic (17), and Mediterranean (8), to investigate the validity of the taxonomic division of the genus into C. maenas (Atlantic) and C. aestuarii (Mediterranean). Consideration of carapace width to carapace length and carapace width to carapace depth ratios, and a set of 10 biometric characters by canonical variates analysis, showed that there were two different statistical populations in the European data set apparently corresponding to Atlantic and Mediterranean specimens of Carcinus. There was still overlap of individuals between the Atlantic and Mediterranean data sets, such that individual crabs could not be assigned unequivocally to one group or the other on the basis of shape. The presence or absence of setae on the frontal distal margin of the cheliped carpus varied geographically. Mediterranean crabs possessed prominent setal brushes, while crabs from the Atlantic north of (and including) Porto in Portugal lacked these setae. Crabs from the other Portuguese sites, from the Palmones estuary in southern Spain, and from Gibraltar did, however, sometimes have a few setae on the cheliped carpus. This observation indicates lack of complete genetic separation of the Atlantic and Mediterranean populations of Carcinus, with the potential existence of a hybrid zone between them. The morphological data do not support the separation of European Carcinus into two taxa distinct at the species level, but additional genetic data are needed to verify the presence of any real gene flow and a hybrid zone between Atlantic and Mediterranean crab populations.