A laboratory model system was developed to investigate the progressive colonization of urinary catheters by Escherichia coli Providencia stuartii, prominent organisms in the polymicrobial bacteriuria of the long-term catheterized urinary tract. We hypothesized that colonization of the catheter and artificial urine by E. coli is influenced by the presence of P. stuartii. E. coli or P. stuartii in pure culture both rapidly colonized the artificial urine and catheters, and both persisted throughout all experiments. In systems containing both organisms, P. stuartii occurred in significantly higher numbers in the artificial urine and on the catheters than E. coli (p < 0.05). To obtain similar numbers of E. coli and P. stuartii in the artificial urine, citrate was eliminated; however, P. stuartii still dominated on the catheter surfaces. The presence of P. stuartii appeared to facilitate growth of E. coli in the artificial urine, yet reduce numbers of E. coli on the catheter. In a separate experiment using different strains of E. coli and P. stuartii, the latter was dominant in the artificial urine and on the catheter surfaces. However, this strain of P. stuartii (which was urease positive) did not facilitate growth of E. coli. The interaction between these strains may have been considerably affected by urea hydrolysis, which resulted in an increase in pH (6.5 to > 8.5) and considerable precipitate formation in the model system. The paradox of P. stuartii enhancing colonization by E. coli in the artificial urine, yet inhibiting its colonization on the catheter surface, illustrates the complexity of polymicrobial interactions in colonization of the catheterized urinary tract.