When introduced as pairs into irradiated, sterile soils, a Pseudomonas fluorescens strain prevented optimal colonization by a P. putida strain. The addition of P. putida to sterile soil already populated by P. fluorescens impeded growth of P. putida in that soil. However, adding P. fluorescens to soil populated with R. putida did not prevent growth of P. fluorescens and caused a decrease in the titer of P. putida. This "preemptive colonization/exclusion" phenomenon was also observed between two isogenic strains. The degree of exclusion depended on the amount of time between the inoculation of the two strains and which organism was introduced first. These results suggest competition for similar "niches" in soils and a hierarchy of fitness among pseudomonads sharing a soil environment. Autoclaving of the soil removed the competitive colonization between Pseudomonas spp. observed in irradiated soils. The competitive colonization of irradiated soils by these two Pseudomonas sp. was reflected in their relative survival when introduced into nonsterile soil.