Pseudomonas aeruginosa introduced directly into the stomach established itself more regularly in the intestinal tract of irradiated mice than of unirradiated mice. The irradiated mice had been exposed to 550 r total body x-irradiation 5 days before inoculation. This difference between irradiated and unirradiated mice could not be explained by differences in pH of gastric or duodenal contents, propulsive motility of the intestinal tract, or the ability of the intestinal contents to support growth of Pseudomonas in vitro.