Polychaetes in soft-bottom intertidal habitats of the Mar Chiquita coastal lagoon (38°00¹S, 57°30¹W, Argentina) are the prey of shorebirds, fishes and crabs. Based on 3 yr of field sampling, we observed that the fire ant Solenopsis richteri is another important predator of polychaetes (mostly Laeonereis acuta) during the summer (December to March). Ants use intertidal areas adjacent to terrestrial habitats which have well-developed soils and are characterized by inland vegetation (i.e. pampas grasses), but appear to be excluded from habitats with the burrowing crab Chasmagnathus granulata. Ants build intertidal galleries perpendicular to the shoreline that are used to transport polychaetes to the nest. The ants attacked the polychaetes in burrows with diameters larger than their own abdomen. They selected larger polychaete sizes, attacking up to 66% of the polychaetes in the high intertidal. Based on our observations, and given the wide range of this species and related fire ants, we believe that this terrestrial-marine interaction between fire ants and polychaetes could be a common phenomenon.