To explore different ideas in relation to the phylogeny of the monocotyledons, in particular about the Agavaceae sensu Hutchinson (1934, 1959) and their relationships with related groups of the Asparagales, the DNA sequence of the chlorplast gene rbcL for 134 species (118 monocotyledons, 16 dicotyledon paleoherbs as an outgroup) was analyzed . Parsimony, distance (neighbor-joining and UPGMA) and maximum likelihood analyses were perform ed. The phylogenies show that Acorus calamus is the living

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... lant more closely related to the ancestral monocotyledons. The different analyses identify three main evolutionary linages in the Agavaceae sensu Hutchinson: 1) One including the Agavaceae sensu stricto, (Agave, Manfreda, Beschorneria, Hesperaloe and Yucca) with Hasta (Funkiaceae) and Chlorophytum (Anthericaceae). 2) Other group that includes the Nolinaceae (Nolina, Beaucarnea and Dasylirion), Sansevieria (Dracaenaceae) and Danae (Ruscaceae). 3) A third group formed by the Asphodelaceae (Aloe, Haworthia and Kniphofia) and Dianella (Phormiaceae). These lineages include the majority of the Asparagales, as were defined by Huber (1969) and Dahlgren et al. (1985). A molecular clock was also estimated to obtain the divergence times among the groups related to the Agavaceae. A rate of 0.34x10-9 nucleotide substitutions per site per year was obtained. Using this calibration, a date for the origin ofthe true Agavaceae (separation between Agavaceae-Hosta) was estimated to be about 14 million years, for the separation of the Agavaceae-Nolinaceae of about 47 million years and for the origin of the Asparagales + Iridaceae + Cyanastraceae of about 84 million years. Nevertheless, these estimates should be regarded as preliminary.