In a densely populated pasture near Hammond, La.,Solenopsis invictaBuren mounds have an aggregate distribution with an average mound density of 0.0096/m. Only 2 other ant species,Monomorium minimum(Buckley) andParatrechina arenivaga(Wheeler), were common. A native fire ant,S. xyloniMcCook, occurred in the soil under several large trees.Random distribution of baits in the field showed that, during daytime, 92–96% of the baits was first discovered byS. invicta, but 100% of them was exploited by this species. On the other hand, only 70% of the baits distributed on theS. invictamounds was exploited byS. invictaitself, while the remaining 30% was exploited mainly byM. minimum. Monomoriumworkers appeared to possess a powerful poison which, when spread in the air, had a strong repellent effect againstS. invictaworkers. In this way Monomorium was often successful even in direct competition for food withS. invicta. This aggression-defense mechanism seemed to be ineffective against the native fire ant,S. xyloni, although the behavior ofMonomoriumto both species of fire ants appeared to be exactly the same and the responses of both species of fire ants seemed identical.