The results of a DNA sequencing study of cycads conducted at the new molecular systematics laboratory at Fairchild Tropical Garden are presented and assessed with reference to previous phylogenetic analyses and classification schemes based on morphology and anatomy. Cycads are an ancient group of gymnosperms that were abundant and widely distributed during the Mesozoic, but are now largely confined to isolated tropical and subtropical regions. The 11-12 genera of cycads currently recognized are thought to comprise a monophyletic group, classified as a single order, the Cycadales, which is divided up into three or four families. Analysis of several chloroplast (trnL intron, trnS-trnG, psbB-psbF, atpB-rbcL) and nuclear gene (ITS2) spacer sequences suggests a number of relationships, some of which were inferred by previous morphological studies, some of which are new. The sequences of Cycas are the most divergent among cycads, suggesting the longest isolation. Dioon is relatively isolated from the other genera, and contains two major clades. Stangeria does not appear closely related to Bowenia, but does seem to have an affinity with Zamia and Microcycas. Lepidozamia is more closely related to Encephalartos than to Macrozamia. Sequence variation among the species of Ceratozamia is low. Microcycas and Zamia are closely related.

Key words: Cycadales, cycads, molecular systematics, phylogeny