A phylogenetic analysis of the monocotyledons was conducted, using four character sets (morphology plus nucleotide sequences of the plastid-encoded gene rbcL and the mitochondrion-encoded genes atpA and cob). The taxon set comprises 58 dicot and gymnosperm outgroups, and 155 exemplar monocots, representing all major lineages of the monocots. The deepest branch within the monocots is between a clade that includes Acorus and all sampled elements of Alismatanae and Aranae, and a second clade that includes all other monocots. Within the latter group there are four lineages of Lilioids, plus a Commelinid lineage that includes all remaining monocots. Within the Commelinids, Arecales are sister of a clade that includes all other members of the group. Morphological characters that contribute substantially to the overall phylogenetic structure include presence/absence of oxalate raphides, vessel elements in leaves, and cell wall ferulates, plus variation in phyllotaxis, leaf venation, stomatal type, microsporogenesis, placentation, endosperm storage constituents, embryo form, and modification of the first tepal as a bracteole.

Key words: atpA, cob, monocots, morphology, phylogenetics, rbcL