Nests of ants are inhabited by many organisms that are specialized for association with ants. Vast majority of such inhabitants is arthropods, and examples among vertebrates are relatively scarce. Our radio-telemetric research on a Madagascan snake, Madagascarophis colubrinus, in northwestern Madagascar demonstrated that M. colubrinus frequently shelters in nests of a particular species of ants, Aphaenogaster swammerdami. When six species of syntopic snakes were presented at the nest entrance, A. swammerdami was least aggressive toward M. colubrinus. In our experiments where chemical cues of potential prey were presented to snakes, M. colubrinus responded to the chemical cues of an ant-eating blindsnake with frequent tongue flicks and/or an open-mouth attack, indicating that M. colubrinus perceives the blindsnake as prey. The blindsnake is a major predator of A. swammerdami, which intrudes ant nests and preys on a large number of brood, but A. swammerdami would be able to reduce the risk of predation by sharing the nest with the predator of the blindsnake. Our results suggest a unique, mutualistic relationship between the ant and the snake, where the host provides a favorable sheltering habitat, and the guest may serves as a ‘guardian’.