Six species of Paraguayan didelphid marsupials (opossums) were described by the Spanish naturalist Félix de Azara (1742–1821), whose nontechnical accounts were the basis for Latin binomials in current use for several species. Unfortunately, some of Azara's descriptions are problematic and have long been a source of taxonomic confusion. In particular, the names currently used for two species of Thylamys are difficult to reconcile with Azara's text. Herein we review the nomenclatural status of Azara's opossums and discuss relevant issues that affect binomial usage. Among other results, we report the rediscovery of the holotype of Caluromys lanatus (Olfers, 1818), and we designate neotypes to preserve current usage of the names Thylamys macrurus (Olfers, 1818) and T. pusillus (Desmarest, 1804).