Populations of cuttlefish (Sepia officinalis) in the Bay of Biscay and the English Channel have previously been regarded as separate populations, since they display different life cycles and different physiological characteristics. pH analyses of in vitro blood oxygen binding capacities show different pH optima for haemocyanin of animals from the Bay of Biscay and the English Channel.However, analysis of seven microsatellite loci carried out on cuttlefish from sites in the Bay of Biscay, the English Channel and the southern North Sea revealed that all individuals appear to belong to a large panmictic population with a low degree of genetic variation, indicating regular genetic exchange. This is substantiated by molecular data for haemocyanin, which indicate the existence of several isoforms throughout the population differing in isoelectric points. The physiological and ecological relevance of the existence of various haemocyanin isoforms within a panmictic population is discussed.