New crossing and selfing experiments within Achillea millefolium agg. (Compositae) succeeded with tetraploid individuals of A. ceretanica-4x, A. collina, A. pratensis and Fl hybrids. Chromosome numbers of 105 out of 785 individuals including F-1, BC, S-1 offsprings and free pollinated Fl progenies were studied. Generally, plants were tetraploid, but occasionally aneuploids and a near-pentaploid plant originated from crossing and selfing experiments with A. ceretanica-4x and A. collina. The high degree of selfincompatibility within Achillea millefolium agg. is confirmed.