A yarn in the form of a supply package in which the yarn is wound on a bobbin is continuously washed with an aqueous washing liquid by standing the yarn supply package upright in means for receiving the yarn package and an aqueous washing liquid, and continuously drawing off the yarn upward from the package while bringing the aqueous washing liquid into contact with the outer surface of the package. The contact of the outer surface of the package with the washing liquid is effected by immersing the package in the washing liquid bath or by flowing the washing liquid down over the outer surface of the package.