Successful crossing experiments between the tetraploids Achillea ceretanica-4x, A, collina, A. distans subsp. styriaca, A. pratensis, and the octoploid A. monticola, all from the A. millefolium agg. (Compositae), are reported. Chromosomes were studied in 514 out of 883 hybrid plants obtained from 13 crossings. F-1-offspring from tetraploid parents was generally tetraploid, but some combinations produced up to 30% of aneuploid, near-pentaploid, and even hexaploid F-1-individuals. A backcross and a F-2-progeny consisted of normal tetraploids. A reciprocal F-1-combination involving a tetraploid parent with one B-chromosome showed an increase of B-chromosomes in the F-1, 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 F-1-cross between tetraploid and octoploid parents.