We studied the structure of an ant community along a 900-m transect of grassland in Argentina. Two ant species, Solenopsis richteri Forel and Camponotus punctulatus Mayr, monopolized space and food. These two species were distributed as a mosaic with most of the transect always controlled by either one or both. Within monospecific areas, each species reached mound densities comparable to those seen where each is an invasive species. Sampling with tuna and sugar water revealed that these two species together monopolized 95% of all baits. Where they co-occurred, S. richteri recruited more effectively and controlled more baits than C. punctulatus. In each area where S. richteri or C. punctulatus was most abundant, each dominant species arrived first at baits but did not differ from the other species in their persistence at these baits. Both species seemed to prefer to forage in the mornings in areas where each was most abundant; however, only C. punctulatus retained this pattern in the areas at which both species had similar densities. The rare occurrence of Pseudacteon parasitoids, and the susceptibility to flooding in the area, may explain the abundance of S. richteri and C. punctulatus, respectively, in this grassland.