The mechanisms of interspecific competition among an invasive and two native Solenopsis fire ant forms were investigated in a series of laboratory experiments. In separate trials each with a different food resource, the native S. geminata×xyloni retrieved the greatest amount of a protein- and lipid-rich artificial food resource and a high protein natural food resource, and the native S. geminata retrieved the greatest amount of a high carbohydrate food resource. In trials investigating aspects of interference competition at the colony level, the invasive S. invicta proved to be initially more aggressive than S. geminata, but less aggressive than S. geminata×xyloni. Solenopsis invicta eventually controlled more of the foraging arenas against both native forms when colonies were equivalent by worker biomass, but not when colonies were equivalent by worker number. When paired with S. invicta, S. geminata suffered a significantly greater proportional reduction in both workers and entire colonies when colonies were initially standardized by worker biomass, but not when colonies were standardized by worker number. When paired with S. invicta, a significantly greater proportional reduction of workers occurred in S. geminata×xyloni, regardless of how colonies were standardized. In pairwise trials at the individual level, majors always exhibited significantly less mortality than minors, regardless of the Solenopsis form. The majors of both native forms suffered significantly less mortality than those of S. invicta. Superiority in colony-level interference ability appears to be an important mechanism allowing S. invicta to displace native Solenopsis forms. The ability of S. invicta to reach high population densities, because of intrinsic biological characteristics or an escape from natural enemies, plays an important contributory role. Similar mechanisms may underlie the success of other invasive ant species.