A device for drawing down an optical fiber preform (2) connected at one end to a feed rod (3) having an equal or smaller diameter. A chamber (11) in which the preform is drawn has an upper part (11A) with an opening (12). The opening engages the preform to seal the chamber. An enclosure (5) has a seal (6) at its upper part (5A) for sealing around the feed rod. A lower part (5B) of the enclosure contacts the upper part of the chamber to seal the chamber while the feed rod is passing through the opening.