Osmorhiza Raf. (Apiaceae) consists of ten species disjunctly distributed in temperate Asia (1 sp.) and the Americas (9 spp.). Osmorhiza berteroi DC. and O. depauperata Philippi are disjunct between North and South America. Within North America, these two species are disjunctly distributed in eastern and western North America and the Great Lakes regions. Phylogenetic analysis was conducted to clarify the interspecific and intraspecific relationships of based on sequences of the ITS regions of nrDNA and ndhF chloroplast gene. With Myrrhis as the outgroup, the monophyly of Osmorhiza is strongly supported. The ITS phylogeny suggests the basal position of the Asiatic O. aristata and the monophyly of the nine New World species. Osmorhiza aristata is relatively divergent from other species even though it is morphologically similar to the eastern North American O. claytonii and O. longistylis. Osmorhiza claytonii and O. longistylis form a clade. Osmorhiza brachypoda, O. depauperata, O. occidentalis and O. purpurea constitute a monophyletic group. The seven populations of the widespread O. berteroi form a clade, which showed no or little sequence divergence, suggesting recent colonization of these widely separated regions by this species. Disjunct populations of O. depauperata from the Rocky Mountains and eastern North American have similar ITS profile and form a clade. Osmorhiza occidentalis, however shows a high level of infraspecific sequence divergence. The sequence divergence of ndhF gene within Osmorhiza is low. Congruent with the ITS data set, the ndhF phylogeny suggests that O. aristata from Asia is basal within the genus. The remaining nine from the New World form a monophyletic group. Osmorhiza glabrata and O. depauperata form a subclade, whereas the ITS phylogeny places O. glabrata basal to the New World clade. The ITS and ndhF phylogenies suggest rapid diversifications of Osmorhiza in western North America.

Key words: Biogeography, ITS, Monophyly, ndhF sequences, Osmorhiza, Phylogeny