This is the first study to document the natural history of Lemur catta, ring-tailed lemurs, outside of gallery forest environments: in xerophytic Didiereaceae-Euphorbia, bush habitat that represents over 90% of this species' potential range. Moreover, this study is the first to document ring-tailed lemurs, or any primate species for that matter, as cactus dwellers. Data were collected for 15 consecutive months on two different troops in the Cap Sainte-Marie (CSM) area, which is Madagascar's southernmost region. All-day follows, nutritional analyses, and health assessments were the methods used to test hypotheses on the following themes: (a) behavioral flexibility, (b) ecology, health, and nutrition, (c) the shared use of space with humans, and (d) the utilization of nonnative species. Significant findings include: extremely large home ranges, highly social and spatially cohesive (juvenile centered) groups, coexistence with Antandroy villagers, and diverse diets comprised of native species despite the population's adaptation to Opuntia monacantha and Opuntia stricta. Based on these findings, I suggest that L. catta have historically been present in the CSM region, but that this population likely persists here today only because of the presence of Opuntia.