The chromosome numbers and morphology in 92 populations belonging to 49 species and three varieties in the genus Delphinium L. (Ranunculaceae), mostly from the Hengduan Mountains region of south-west China, were studied. Forty seven species and three varieties were diploid, with 2n = 16, one species was tetraploid, with 2n = 32, and one species had diploid and tetraploid cytotypes. Three species had B chromosomes, representing the first time the occurrence of B chromosomes has been reported in the genus. The karyotypes of all the diploid species were quite uniform, commonly bimodal, and usually consisted of one pair of large median-centromeric (m), one pair of large submedian-centromeric (sm), five pairs of medium-sized subterminal-centromeric (st), and one pair of smaller sm (rarely st) chromosomes. The low incidence of polyploids in Delphinium from the Hengduan Mountains region indicates that polyploidy has played a minor role in the speciation of this highly diversified genus in the region. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158, 172–188.