1. When entering freshwater, young European eels either become resident or disperse to settle upstream. While this process shapes the whole population structure, little is known about how the decision' is made. Here, we investigated the possible role of body size. 2. A 1-year capturerecapture programme was carried out on 1505 European eels (length-range 62633 mm) at two freshwater sites on the Saucats River, France. Eels were separated into two groups based on body length and this determined the tagging method; 1350 smaller eels (239 mm) were tagged individually with a passive integrated transponder. 3. Migration was detected only in eels 0.05 means that such eels contribute substantially to upriver colonisation, this corresponds to eels