The red imported fire ant, Solenopsis invicta Buren, is exotic to Mexico. It has been detected recently at least 3 km south of the Texas-Mexico border, in Matamoros, Tamaulipas, Mexico. Previous studies in the USA have indicated that the arrival and establishment of the red imported fire ant has resulted in reductions of density and diversity of resident ant populations; S. invicta also modifies the associations with and among other ant species. This initial ecological study was conducted from 2007 to 2009 in Matamoros, Mexico, to assess the impact of the red imported fire ant on relative density, diversity, and interspecific associations of other resident ants. The goal of the study was to determine the impact of the imported fire ant infestation on remaining resident ants in two habitat types: (agricultural fields and natural habitats) in adjacent S. invicta infested and non-infested areas. Pitfall traps were used to estimate ant abundance and diversity. A total of 31,838 ants (12,515 in the non-infested area and 19,323 in the infested area) was collected, distributed among 26 genera in six subfamilies. The most abundant taxa in both areas (infested and non-infested) were the red imported fire ant and Pheidole spp., respectively. In both areas, the prevailing associations were generally constituted by Pheidole spp., Forelius spp., the red harvester ant, Pogonomyrmex barbatus Smith, and the pyramid ant, Dorymyrmex spp. These ants are positively associated among them, but they are strongly and negatively associated with the red imported fire ant, as reported in previous studies. There appeared to be a general shift in dominance with S. invicta often antagonizing and displacing the previously dominant ants. Diversity of ants was not different among S. invicta-infested and non-infested areas, but non-S. invicta ants abundance was lower in infested areas.