Successful crossing experiments between tetraploid individuals of Achillea ceretanica, A. collina, A. distans ssp, styriaca, and A. pratensis (A. millefolium complex, Compositae) were carried out. Sesquiterpenes of 1013 hybrid plants obtained from 13 crossings were studied by qualitative thin layer chromatography (TLC). F-1-progenies from parent plants with guaianolides (GU-type), as well as the offspring of a back-cross, uniformly contained proazulenes. Diallel crosses between plants exclusively wi th eudesmanolides (EU-type) resulted in a 7:1 segregation-ratio of EU- and GU-plants. Two reciprocal crossing experiments between EU- and GU-parents resulted in 28.6% and 89.3% EU-individuals, as well as GU-offspring. According to these findings, the qualitative marker 'proazulene' (GU-type) is thought to be a recessive character of the tetraploid Achillea species studied.