Population substructure of the cuttlefish Sepia officinalis (Mollusca: Cephalopoda), as investigated by genetic variation of microsatellite loci, has been reported to be extensively variable around the Iberian Peninsula (Pérez-Losada et al., 2002) and panmictic in the semienclosed Adriatic Sea (Garoia et al., 2004). Yet, no verified genetic information on population substructuring existed for the northern distribution range of this species in the English Channel and Bay of Biscay.So far, reproductive and migration behaviour and in-vitro oxygen binding properties of haemocyanin have suggested separate populations in the English Channel and the Bay of Biscay. However, extensive examination of genetic variation at seven microsatellite loci indicated no significant differences between cuttlefishes from both areas. The revealed degree of genetic variation at the screened loci is comparable to the panmictic population of S. officinalis in the Adriatic Sea (Garoia et al., 2004). S. officinalis in the English Channel and the Bay of Biscay should thus also be regarded as a panmictic population.Garoia, F et al. (2004). Heredity, 93: 166-174.Pérez-Losada, M et al. (2002). Heredity, 89(6): 417-424.