Tribe Eriachneae in subfamily Micrairoideae is one of the least explored of the ca. 22 C4 lineages in the grass family (Poaceae). Whereas many C4 lineages are more species-rich, more morphologically disparate, and wider ranging than their C3 sisters, Eriachneae has fewer species, less disparity, and covers a far smaller geographical area than its C3 sister tribe Isachneae. Tribe Micraireae, which is C3 and sister to Eriachneae and Isachneae, occupies habitats more similar to C4 Eriachneae than C3 Isachneae. Evolutionary analyses within the subfamily are hindered by the lack of a phylogenetic framework for any substantial sample of species. Additionally, only a handful of members of Micrairoideae have been tested for photosynthetic pathway. This study presents the first well-resolved phylogeny of Eriachneae based on full plastome sequences from almost half of the species in the tribe. Photosynthetic pathway is tested for 47 species representing all three tribes of Micrairoideae using carbon isotopes to test the assumption that C4 is restricted to Eriachneae. Habitat preferences among the tribes are estimated using bioclimatic data, with multivariate analyses showing that habitats of Micraireae and Eriachneae are more similar to each other than either is to Isachneae, and the range of environments is greater in Isachneae than in Eriachneae. All measured Eriachneae are confirmed to be C4, and all Isachneae are C3. Evolutionary interpretation of these results is necessarily preliminary, and greater phylogenetic sampling in Isachneae is needed to estimate diversification rates and ancestral habitats, but subfamily Micrairoideae appears to be an interesting exception to general patterns of C4 evolution in grasses.