Presence of Achillea roseoalba Ehrend. in the Pyrenees Achillea roseoalba Ehrend., a perialpine species of the A. millefolium aggregate, is recorded for the first time in the Pyrenees and the Iberian Peninsula. We have localised some populations in a small area of the upper basin of the Segre river around Seu d'Urgell town, in highly anthropized hay meadows at an altitude of 600-750 m a.s.l., where it is locally common. In the Pyrenees, it can be considered an alien species, probably introduced in the first half of the 20th century due to activities related with dairy farming. We also presents a brief morphological characterisation of the plant, compared with A. ceretanica Sennen and A. millefolium L. s.l., the other two species of the aggregate present in the Eastern Pyrenees.