Camponotus schmitzi is the sole plant-ant to be associated with a carnivorous plant. It lives specifically in the tendril of Nepenthes bicalcarata, a climbing plant from Borneo bearing pitcher traps that capture and digest arthropod prey. The nature of this intriguing ant-plant association was not clearly elucidated. The ant has first been described as unaggressive and suggested to benefit its host plant by preventing pitcher putrefaction through the selective removal of large but dead prey items. It was then shown to provide protection against a specific weevil feeding on pitcher buds. We thus tested whether Camponotus schmitzi also displays aggressiveness against living insects on mature pitchers and whether it hampers or rather favours prey capture in Nepenthes bicalcarata. Using bioassay and video, we revealed that Camponotus schmitzi exhibits a hunting behaviour that had remained cryptic. We showed that, unlike most specific plant-ants which conspicuously and dissuasively patrol their host-plant, the ants spend most of their time concealed, letting numerous visitors, mostly ants, access the nectar produced by the external parts of the pitcher. They lie in ambush under the pitcher rim until visitors fall into the pitcher. Camponotus schmitzi then systematically attack these prey items attempting to escape from the pitcher. We demonstrated that pitchers occupied by Camponotus schmitzi retained more experimentally-dropped ants than Camponotus schmitzi-deprived pitchers. We also showed that C. schmitzi regularly feeds on prey items inside the pitchers. Therefore the ant helps the plant to catch and maybe also digest its prey. These results suggest that the ant-plant interaction is a nutritional mutualism, implying the unusual association of carnivory and myrmecotrophy