Phylogenetic analysis of sequences of the ITS and ETS regions of 18S-26S nuclear rDNA for all genera of Stephanomeriinae (sensu Stebbins) except Thamnoseris and all of Microseridinae (sensu Stebbins) reveals that both subtribes are unnatural and identifies a series of subclades, each with very strong bootstrap support (greater than 96%). Subclade 1: Munzothamnus, Pleiacanthus, Rafinesquia, and Stephanomeria, all with x=8, and Prenanthella with n=7. Subclade 2: Agoseris, Microseris, Nothocalais, Uropappus, and Stebbinsoseris. Subclade 3: Lygodesmia (sensu Tomb), Chaetadelpha, both with n=9, and the annual Shinnersoseris with n=6. Subclade 4: the recently discovered monotypic Marshalljohnstonia, a rosette-shrub from northern Mexico, and Pinaropappus, a genus of perennial herbs also found in Mexico. Several additional, well-supported subclades can be recognized, each including some species of Malacothrix (sensu lato), a genus now seen as polyphyletic with quite disparate elements. Partitioning of the two subtribes into smaller subclades greatly simplifies the relationships among these genera. Considered as a group, the 24 genera examined represent a single, major radiation of Lactuceae mostly confined to western North America.

Key words: Compositae, ETS, ITS, Lactuceae, Microseridinae, Stephanomeriinae