Successful crossing experiments between the tetraploidsAchillea ceretanica4x, A. collina, A. distanssubsp.styriaca, A. pratensis, and the octoploidA. monticola, all from theA. millefoliumagg. (Compositae), are reported. Chromosomes were studied in 514 out of 883 hybrid plants obtained from 13 crossings. Fi-offspring from tetraploid parents was generally tetraploid, but some combinations produced up to 30% of aneuploid, near-pentaploid, and even hexaploid F1-individuals. A backcross and a F2-progeny consisted of normal tetraploids. A reciprocal F1-combination involving a tetraploid parent with one B-chromosome showed an increase of B-chromosomes in the F1, when the male transmitted the B (69.2%), and a decrease, when the female had the B (33.3%). Mainly hexaploid offspring resulted from a reciprocal F1-cross between tetraploid and octoploid parents.