The subgenus Marmosa (Micoureus) Lesson, 1842 includes six species of long-tailed, black masked mouse-opossums widely distributed in forested areas of the Neotropics from northern Argentina to Belize. Most of the nominal forms of Marmosa (Micoureus) have not been revised since 1933 and some currently accepted synonymies are in need of revision; similarly distributions of these forms remain for the most part unclear. Herein, we report Paraguayan new and noteworthy locality records for Marmosa (Micoureus), including the first records for the western Dry Chaco region. Specimens were identified to the species level on the basis of morphological and molecular data. In addition, we conducted a phylogenetic analysis that includes sequences of five of the six species currently recognized of Micoureus incorporating a total of 70 sequences of the subgenus. This constitute the most taxonomically and geographically dense phylogenetic analysis of Micoureus. Results show that the most basal dichotomy of the Micoureus clade does not delimit cis- and trans-Andean reciprocally monophyletic groups, rending the cis group paraphyletic to the single trans-species included, suggesting that the colonization of the western (trans) side of the Andes was a relatively late event in the biogeographic history of Micoureus. In addition, the phylogenetic analysis shows that additional taxonomic work is much needed to clarify the number of distinct biological units, either species or subspecies, within Micoureus. (c) 2011 Deutsche Gesellschaft fur Saugetierkunde. Published by Elsevier GmbH. All rights reserved.