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The PACCAD clade (Panicoideae, Arundinoideae, Chloridoideae, Centothecoideae, Aristidoideae, Danthonioideae) is well supported in all recent studies of Poaceae. However, phylogenetic relationships within the clade are still unresolved, although several branches are strongly supported. In this study, we focus on the phylogeny of the Centothecoideae + Panicoideae clade, one of the major lineages within the PACCAD clade, and one of the most diverse in the family. Two previously unsampled tribes, Isachneae and Steyermarkochloeae, are included, as are additional taxa of Centothecoideae. Phylogenetic analyses of plastid genome sequences (ndhF gene and rpl16 intron) and structural data show increased support of the centothecoid–panicoid clade, but phylogenetic relationships between the two putative subfamilies remain unresolved. Centothecoideae may be sister to Panicoideae or they may be paraphyletic with respect to Panicoideae, as monophyly of Centothecoideae is weakly supported at best. Polyphyly of Panicoideae is demonstrated as Isachneae and Steyermarkochloeae (only with ndhF) form well-supported clades with Micraira–Eriachne and Chasmanthium, respectively. Polyphyly of Arundinelleae is also confirmed as Danthoniopsis and related genera resolve elsewhere. Centothecoideae, as recently circumscribed, resolve as three strongly supported clades including Danthoniopsis and related genera. Cyperochloa (previously incertae sedis) and arundinoid Spartochloa are sister and fall within the centothecoids with strong support. Centotheceae are polyphyletic and segregate into three major groups. One centothecoid genus, Zeugites, is demonstrably paraphyletic.