1 Hatchery‐reared Pacific oysters (Crassostrea gigas) grown in the British Isles and Guernsey originate from an introduction through quarantine of small numbers of oysters from British Columbia, Canada.
2 Previous electrophoretic analysis of samples of these oysters have shown the glucose phosphate isomerase locus to be monomorphic.
3 Recent analysis of oysters from a commercial hatchery in Guernsey revealed a second allele at low frequencies which was unexpected and was thought to indicate the presence of oysters from another stock.
4 Samples of hatchery‐reared oysters from France showed that they were highly polymorphic and carried the unexpected allele and four others.
5 A natural spatfall of oysters from the River Teign (Devon) was sampled and these carried three alleles which were also found in the French C. gigas. This suggests that there has been movement of C. gigas possibly of French origin into this river.
6 This finding and its implications in terms of conservation and management are discussed.
