Cycadales, the cycad order, has approximately 185 extant species in eleven genera and three families (Cycadaceae, Stangeriaceae and Zamiaceae) (Jones 1993). Cycads are confined to the tropical and subtropical regions of both hemispheres. They were shown to be the basal-most clade of living gymnosperms based on multiple gene analyses (Chaw et al. 2000; Bowe et al. 2000), but relationship of their families and genera are not completely known. In this report chloroplast matK gene is used to infer the phylogeny among the genera and higher taxa of cycads. Ginkgo is employed as the outgroup. Analysis using either the nonsynonymous sites or the first two position of matK gene identifies Cycas as the basal-most group of cycads, Stangeria as the next basal, and the rest genera form a well supported group, in which three subfamilies, Bowenioideae, Encephalartoideae, and Zamioideae as defined by Stevenson (1992), are each resolved as monophyletic. Traditionally the latter two subfamilies constitute the family Zamaiaceae, which, howevevr, is not sustained by the matk data. In contrast to morphological and cytological studies, our data do not support inclusion of Bowenia in the family Stangeriaceae. In addition, Lepidozamia is shown to be sister to Encephalartos rather than to Macrozamia. The nonsynonymous sites of matK gene in cycads (except Lepidozamia) have evolved in a relatively clock-like manner.

Key words: Cycadaceae, Cycadales, evolution rate, Phylogeny, Stangeriaceae, Zamiaceae