Abstract: A wasp of the genus Ampulex (Hymenoptera: Ampulicidae) deprived ants [Hymenoptera: Formicidae: Ectatomma ruidum (Roger, 1860)] of liquid food that they had collected in a tree canopy in Costa Rica. In a first quick attack, the wasp appeared to examine whether a given ant worker was carrying a droplet between the mandibles. In a second more pronounced attack only aimed at laden workers, she took the droplet over from the ants. In this unique observation, the wasp was effective in stealing the food, as less than 10 % of workers carrying liquid food successfully reached the colony.