In fall 2009, six migrating silver American eels (Anguilla rostrata) originating from a stocking program were caught in the brackish waters of the St. Lawrence Estuary. These maturing eels were all females with gonads developed at a similar stage as other migrating eels in the estuary. Fluorescent Oxytetracycline marks observed on the otoliths allowed us to assert without any doubt that they came from glass eels caught in Nova Scotia and stocked 4 years earlier in the Richelieu River, 500 km upstream from the recapture location. Their length varied between 570 and 668 mm, which is within the size range of naturally recruited female silver eels in Nova Scotia, while silver eel are most generally longer than 80 cm in the St. Lawrence estuary. Their growth rate was also exceptionally fast. This direct observation is the first evidence that American eels stocked as glass eels can migrate seaward at least as far as the estuary in synchrony with naturally recruited female silver eels en route to their spawning grounds in the Sargasso Sea.