There are only a small number of molecular markers currently proven to be useful for phylogenetic inference within the flowering plants. We demonstrate that the 5' end of the 26S ribosomal DNA (ca. 1100 bp) is of great value for investigating generic to subfamilial relationships. We analyzed DNA sequences from 31 species of the Ranunculaceae and four species of the Berberidaceae to test phylogenetic relationships within the Ranunculaceae. The inferred phylogeny strongly supports the concept that the Thalictrum chromosome group is not monophyletic, but consists of three independent lineages: (1) Hydrastis, (2) Xanthorhiza and Coptis, and (3) Thalictrum, Aquilegia, and Enemion. Based on comparison with conventional taxonomic characters, we propose a hypothesis that the third group also includes the rest of the Thalictrum chromosome taxa that have a base chromosome number of seven. For the Ranunculus chromosome group, our study suggests several relationships that have not been recognized by conventional systematics. The inferred 26S rDNA topology is compared with results from two previously published molecular data sets: DNA sequences from rbcL, atpB, and 18S rDNA genes and restriction fragment length polymorphism data from chloroplast DNA. The three topologies are highly congruent and agree with karyological characters, but not with fruit type, both of which have often been used for the higher classification of the Ra- nunculaceae.