The extraordinary invasion of siskins ( Carduelis spinus ) along the Belgian Coast during the 1965-1966 Winter, has been especially studied in the Bird Reserve of the «Zwin» at Knokke (51°2l'N., 3°18'E.) where 5.375 siskins were captured and ringed between October 1965 and May 1966. Out of these, 4.627 were ringed during the two months of December and January.The following subjects are treated :1. Causes of this invasion. Several possibilities are put forward; if these possibilities cumulate, this could explain the invasion. The siskin populations of Northern Europe travel by two different ways to reach our country : the coastal way through the Baltic Plain, and the more continental way, from Scandinavia and Russia via Central Germany, the Belgian Ardennes and the north of France and from there turning Northerly. Those two siskin migrating flocks seem to have met along the Belgian Coast which became a turning point from where those birds continued their journey in several directions, mainly the easterly, in order to reach the woods of Campine and Ardennes.2. Timing of the migration in the «Zwin».3. Origin of these migratory populations, in the light of the foreign rings recovered in Belgium (with map).4. Direction of migration. From the «Zwin» and already since October, the majority of siskins were flying in an easterly direction.5. Proportion of ages and sexes. A majority of a majority of males; juveniles, especially strong with the females.6. Speed of migration. The ringing records give an idea of this speed and also of the instability of the wintering siskins, perpetually on the move, hesitating frequently, as shown by several «go-and-back» travels.7. Other information obtained in Belgium.