The brown seaweed Undaria pinnatifida is recorded for the first time along the Belgian coast. It was found in July 1999 on a pontoon of the marina at Zeebrugge. It concerns a small population of about 30 thalli. Most probably the alga was introduced in Zeebrugge via hulls of yachts coming from the French coast of the Channel where Undaria occurs at several localities after it escaped from commercial nurseries in Brittany. An origin from the South of England cannot be ruled out. Since Undaria has been discovered in 1998 at Calais it may equally have been introduced from there. Our specimens possibly settled end 1998 and grew fast during the subsequent winterperiod 1998-99. At Zeebrugge the thalli of Undaria are usually considerable overgrown with fouling-organisms (tunicata, bryozoa, hydroids, small seaweeds, .), what could facilitate the early decay of the alga' s laminae as observed. Especially the star ascidian Botryllus schlosseri may possibly cause much damage by suffocating other overgrowing organisms. The plants were fertile, which, in the future may provide for spread and settlement of the species in the area.