1. We describe the simultaneous occupation of a rare understorey ant-acacia Acacia mayana by its guarding ant Pseudomyrmex ferrugineus, and an apparent opportunist parasite of the mutualism, the generalist ant Camponotus planatus. The two ant species occur together in 30.7% of the 26 mature A. mayana plants (23.5% of all trees (n = 34)) surveyed, but C. planatus is absent from saplings below 1 metre in height (n = 8). 2. While P. ferrugineus shows behaviour compatible with effective host tree defence, C. planatus does not attack phytophagous insects and appears ineffective as an ant-guard. C. planatus does however occupy swollen thorns (pseudogalls) on the host tree, and harvests nectar from extrafloral leaf nectaries. We propose that C. planatus is a parasite of the Acacia-Pseudomyrmex mutualism. 3. C. planatus does not harvest the second trophic reward produced by the tree for its Pseudomyrmex ant-guards, protein-rich food (Beltian) bodies. C. planatus lack the specialised larval adaptations needed to use Beltian bodies as brood food, suggesting that this resource is potentially more resistant to exploitation by generalists than extrafloral nectar. 4. In competition for access to nectaries, C. planatus effectively displaced P. ferrugineus in 99.8% of encounters. These results suggest not only that C. planatus is a parasite of this mutualism, but also that it is able to effectively counteract the aggression shown to other insects by the resident ant-guards
