In this report, the second of a revisionary series on mouse opossums (Marmosa), we analyze cytochrome b sequence data from 166 specimens of the subgenus Micoureus and delimit putative species using the multirate Poisson Tree Processes (mPTP) method. That analysis identifies 21 putative species, many of which can be matched with available names, including alstoni, constantiae, demerarae, limae, germana, meridae, paraguayana, parda, perplexa, phaea, rapposa, and rutteri. However, some of these nominal taxa are not morphologically diagnosable, and in the absence of other corroborating evidence, we do not recommend that they all be recognized as valid. Phylogenetic analyses of a multigene dataset suggest that putative species of Micoureus belong to several well-supported clades, one of which (the "Rapposa Group") is revised in this report. As defined herein, the Rapposa Group includes at least three valid species: M. rapposa Thomas, 1899 (including budini Thomas, 1920); M. pardaTate, 1931; and M. rutteriThomas, 1924. Herein we document their ecogeographic distributions and diagnostic traits, comment on their taxonomic histories, and list the specimens we examined (including all relevant type material).