Although the identity of the sister group of the Leguminosae has been much debated, recent phylogenetic analyses based on many molecular markers have restricted the candidates to three families, the Polygalaceae, the Surianaceae and the monotypic Quillajaceae. The Leguminosae and these three families form the order Fabales in recent classification systems. The Fabales are a well-supported group but the relationships among families within the order remain unclear. Almost all possible relationships among these four families have been proposed by various studies. We present a phylogenetic analysis of the Fabales based on sequences of the chloroplast rbcL gene and chloroplast trnL intron. The sampling consists of all four genera of the Surianaceae, most genera of the Polygalaceae including several species of the polyphyletic genus Polygala, and Quillaja saponaria, the sole species of the Quillajaceae. The sampling within the Leguminosae is focused on subfamily Caesalpinioideae, found to be basal in the family in other studies, and especially tribe Cercideae for which all five genera are included. Species of closely related orders (Rosales, Cucurbitales, Fagales, Zygophyllales) were included as outgroups. Results show that Quillaja saponaria is sister to the remainder of the order. The Polygalaceae are sister to a clade formed by the Leguminosae and the Surianaceae. Relationships within the Leguminosae are similar to those found by other molecular phylogenetic analyses of the family. Tribe Cercideae is sister to the remainder of the family in which subfamilies Papilionoideae and the paraphyletic Mimosoideae are nested in a paraphyletic subfamily Caesalpinioideae

Key words: chloroplast DNA, Fabaceae, phylogeny, Polygalaceae, Quillajaceae, Surianaceae