This chapter presents a paleoecological analysis of non-carnivorous Santacrucian marsupials of the orders Paucituberculata and Microbiotheria. Different ecological niches are inferred from estimates of body mass, diet, and locomotor behavior. Body masses were estimated using a regression analysis based on living marsupials. Possible dietary preferences were explored by an analysis of the development of molar shearing crests. Inferences about locomotor behaviors of some species with well-preserved postcranial skeletal remains were derived from a published morphofunctional analysis. From the wide range of estimated body masses and diet several niches were inferred: small- to medium-sized insectivores, small- to medium-sized insectivore–frugivores, and medium- to large-sized frugivores. According to our results, Paucituberculata and Microbiotheria of the Santa Cruz Formation constitute an ecologically diverse assemblage that inhabited forested habitats, developed under warm temperatures and seasonal rainfall. These forested habitats could have supported several non-carnivorous marsupial niches, offering diverse resources both in the spatial dimensions and in the trophic ones.