The circumscription of the grass subtribe Calothecinae has undergone several changes since its description. Currently, it comprises Chascolytrum and the recently described genera Laegaardia and Paramochloa.Here we evaluate the circumscription of Calothecinae and the recently proposed infrageneric classification of Chascolytrum based on a phylogeny with more comprehensive taxon and molecular marker sampling than in previous studies. We sampled all Calothecinae genera, all but one Chascolytrum species, two South American Trisetum s.l., and representatives of the subtribes Agrostidinae, Brizinae, Echinopogoninae, Koeleriinae, Phalaridinae, and Torreyochloinae within Poaceae tribe Poeae. We performed Bayesian and Maximum Likelihood analyses of four plastid DNA regions (atpF-atpH, matK, rps16 intron, and trnL-trnF) and two nuclear ribosomal regions (ITS and ETS). Our results revealed that neither Calothecinae nor Chascolytrum is monophyletic, as currently recognized, because Trisetum brasiliense and Trisetum bulbosum are nested within Chascolytrum. We include these two Trisetum species in Calothecinae as incertae sedis. Laegaardia and Paramochloa form a clade that is sister to the Chascolytrum + Trisetum clade, and based on morphological characters, we transfer the former to the new subtribe Paramochloinae. Our Chascolytrum phylogeny is better resolved and supported than in previous studies, and based on these results, we divide Chascolytrum into nine genera, including two new ones: Boldrinia (gen. nov.), Calotheca, Chascolytrum, Erianthecium, Lombardochloa, Microbriza, Poidium, Rhombolytrum, and Rosengurttia (gen. nov.). We provide a key to Calothecinae genera, descriptions of the genera, nomenclatural information, and keys to species of each genus. In addition, six new combinations are proposed.

Chascolytrum serranum, a new species of C. sect. Hildaea restricted to Serra Geral National Park and surroundings, in Cambará do Sul, Rio Grande do Sul, Brazil, is described and illustrated. The new species is morphologically similar to C. ambiguum and C. juergensii var. angustilemma, but is distinguished from them mainly by having paleas more than 2/3 the length of the lemmas (up to 2/3 the length of the lemmas in C. ambiguum and C. juergensii var. angustilemma), glabrous and smooth lemmas (strigose in C. juergensii var. angustilemma), and puberulous paleas (glabrous in C. ambiguum). The species can be recognized in the field by the purplish and shiny spikelets. It inhabits river banks and, less frequently, swamps.