We sequenced the entire coding region of the chloroplast gene matK of 32 Poaceae taxa including 18 genera of subfamily Chloridoideae, 11 genera representing subfamilies Arundinoideae, Bambusoideae, Centothecoideae, Panicoideae, Pooideae, and Oryzoideae plus the sister taxon Joinvillea (Joinvilleaceae) as an outgroup. Cladistic analysis of matK sequence variation using PAUP*4.0 recovered two equally parsimonious trees that are consistent with the maximum likelihood tree. In the strict consensus tree, the bambusoid Sasais sister to the remaining Poaceae taxa followed sequentially by clades representing subfamilies Oryzoideae and Pooideae. Monophyly of the PACC clade (Panicoideae-Arundinoideae-Centothecoideae-Chloridoideae) is strongly supported with a 87% bootstrap value, but relationships among these subfamilies are not well supported. The Chloridoideae appear as a monophyletic lineage supported with a 92% bootstrap value. Although genera in the tribe Unioleae are considered closely related, their phylogenetic position has been questioned. Our results indicate that Uniola plus Fingerhuthia (both Unioleae) are monophyletic and basal in the Chloridoideae, followed by the Sporoboleae, Orcuttieae, and three Chlorideae genera Astrebla, Enteropogon, and Trichloris. The remaining Chloridoideae genera sampled represent the tribes Chlorideae, Eragrostideae, and Pappophoreae. Although members of the two largest tribes, Chlorideae and Eragrostideae, appear polyphyletic, some grouping was evident. The absence of strongly supported groups within the Chloridoideae reflects the lack of agreement on tribal classification in this subfamily.

Key words: Chloridoideae, chloroplast DNA, matK, phylogeny, Poaceae