The grass subfamily Panicoideae includes over 3000 species, approximately 500 of them in the polymorphic and polyphyletic genus Panicum. We have used full-length sequences of the chloroplast gene ndhF to determine the phylogeny of the group. The final data set includes sequences of 102 panicoids plus 6 outgroups; of these 79 sequences are new to this study. The Panicoideae are monophyletic, supporting all previous molecular and morphological studies. The subfamily is divided into three strongly supported clades, representing groups with the same chromosome base number: the Andropogoneae (x=10), x=10 Paniceae and x=9 Paniceae; the first two may be sister taxa, but this result is not strongly supported. The point of origin of C4 photosynthesis is ambiguous, and varies among and within strongly-supported clades. The PCK and the NAD-ME C4 subtypes each originated once and are unreversed synapomorphies for the Urochloa-clade and for Panicum subgenus Panicum, respectively. All other C4 species are NADP-ME and lack an outer bundle sheath; despite their biochemical and anatomical similarity, these do not form a monophyletic group. Panicum subg. Panicum is unrelated to the other subgenera of Panicum. Panicum maximum, the sole species in Panicum subg. Megathyrsus is better placed in Urochloa, as suggested by earlier authors. Panicum subg. Agrostoides, subg. Phanopyrum and possibly also subg. Dichanthelium are polyphyletic. Paspalum is paraphyletic, with Thrasya derived from within it. All genera with bristles in the inflorescence (Setaria, Pennisetum, Cenchrus, Paspalidium) form a clade. Previously defined subtribes are not monophyletic, but some clades in the phylogeny are similar to those in morphological cladograms.

Key words: C4 photosynthesis, ndhF, Panicoideae, Panicum, Poaceae