The most comprehensive phylogenetic study of monocotyledons based on DNA sequences includes 100 of the 102 currently recognized families (APG 1998) and is based on a combined analysis of rbcL, atpB, and 18S (Chase et al. 2000). This phylogeny is well resolved and well supported, except for the higher level inter-relationships among some orders. Separate analyses of the plastid genes, rbcL and atpB, result in well resolved strict consensus trees, whereas an analysis of the nuclear 18S gene gives a far less resolved consensus tree. Furthermore, the composition of a number of clades in the 18S topology differs from those of the plastid gene topologies. It is therefore important to add another nuclear DNA region to the three-gene data set, to gain further confirmation for the plastid DNA topologies and higher bootstrap support for the ordinal clades. Only a few studies have used the 3400 base pair long 26S gene for cladistic analyses, and we explore the phylogenetic information from this nuclear ribosomal DNA region. The results presented are based on the first 1200 base pairs, which contain six of the 12 expansion segments. These sequences could be aligned easily across monocot families, except for a few highly variable areas of the expansion segments. The 26S region produced about the same number of parsimony informative characters as rbcL and atpB, and more informative characters than 18S. The strict consensus tree resulting from cladistic analysis of 26S sequences is comparable to the well resolved strict consensus trees of rbcL and atpB, and much better resolved than the 18S consensus tree. The 26S topology largely recovered the clades present in the previous analyses of rbcL, atpB, and combined rbcL, atpB, and 18S. Many smaller clades received high bootstrap support, but most major clades had less than 50% support.

Key words: 26S nrDNA, molecular systematics, monocotyledons