Passive Integrated Transponder tags (PIT tags) are recommended as the most suitable method for tagging fish on the basis of their high retention and fish survival rates. The objective of this study was to determine the long-term effects (between 1 and 11 years) of the PIT tag on the growth of the yellow European eel (Anguilla anguilla). A difference of more than 50% was observed in the growth of marked and unmarked yellow eels. If this is a general long-term effect in all eels, it would seriously restrict the use of PIT tags for studying the dynamics of European eel populations, and so for their management.