Twenty-seven chromosome counts are reported in 23 species of the genus Centaurea, mostly eastern endemic species of the Jacea group, which has become the core of the re-defined genus Centaurea. Twenty reports are new, one is a correction of a previous count, one is a confirmation of limited previous data and one represents a new basic number in the Centaureinae. The prevalence of the basic chromosome number x = 9 among the Eastern sections of the Jacea group is confirmed, together with the close correlation between karyological data and classification of the genus. Two alternative hypotheses on the aberrant chromosome number (for the Centaureinae) found in C. behen are proposed. One of them, if verified, would confirm that a cycle of polyploidy and descending dysploidy is a key mechanism in the speciation of the group. © 2004 The Linnean Society of London.