One of the potential human health effects associated with the environmental release of microorganisms is colonization of the intestinal tract. The study uses serial transfer techniques to monitor the in vitro survival and competition with human fecal microbiota of Pseudomonas aeruginosa strains BC16 and AC869. Strain BC16 was isolated from a commercial product for polychlorinated biphenyl (PCB) degradation, and AC869 is a 3,5-dichlorobenzoate degrader. In addition, a mouse intestinal isolate, Pseudomonas aeruginosa strain PAMG, was studied as a positive control. Results were compared to previously published mouse fecal microbiota studies. Quantitative comparison of fecal microbiota populations enumerated on selective media found human and mouse cultures to be nearly identical. Survival of the competitor strains was also similar in both the human and the mouse systems. By culture 5 of the human serial transfer experiments, strains AC869 and PAMG were present at significantly higher levels than strain BC16. In previous serial transfer experiments with mouse fecal flora, strain AC869 was present at a higher level than strains PAMG and BC16 by culture 5. No alterations of the microbiota populations due to the addition of a competitor strain were found. The system can be helpful in identifying environmental strains with a high potential for colonizing the intestinal tract.