Phylogenetic analysis of rDNA ITS sequences shows that all of the western North American genera of Lactuceae with a base chromosome number of n = 8, including Stephanomeria Nutt., Munzothamnus P. H. Raven, and Rafinesquia Nutt., constitute a monophyletic group. Stephanomeria, with 17 annual and perennial species, forms a single clade and is more closely related to Rafinesquia than to Munzothamnus. Munzothamnus contains a single perennial species endemic to one of the Californian Channel Islands and has been previously placed both within Stephanomeria and within Malacothrix; our results suggest it is an ancient lineage best recognized as a distinct genus. Rafinesquia contains two annual species in California and the adjacent deserts and its close relationship to Stephanomeria was not previously recognized. The results also show that Stephanomeria spinosa (Nutt.) Tomb, which was transferred to Stephanomeria from Lygodesmia D. Don, is the sister group of either Rafinesquia or Stephanomeria or both and probably is best treated as a monotypic genus, Pleiacanthus Rydb. Two perennial species, Stephanomeria pauciflora (Torr.) A. Nelson and the tetraploid S. parryi A. Gray, are closely related to each other and appear to be more closely related to the six annual species than to the other perennials.

Key words: Asteraceae, Compositae, ITS, Lactuceae, Phylogeny, Stephanomeria