Cercocarpus H.B.K. (Rosaceae) has undergone three revisions, each varying widely in the ranks assigned to taxa and the number of species. Discrepancy among revisions is due primarily to a continuum of morphological variation within and between taxa. We have developed a molecular phylogeny to address this discrepancy. The ITS region of the nuclear ribosomal repeat contained insufficient phylogenetically informative sequence variation, so we developed the external transcribed spacer (ETS) of the nuclear ribosomal repeat. The ETS has more informative variation than the ITS and a unique central region which has shown to be an important marker within this group. The correct taxonomic affiliation of C. traskiae, California's rarest tree, will also be addressed.

Key words: Cercocarpus, ETS, external transcribed spacer, molecular phylogeny, Rosaceae