At the time indwelling urinary catheters were removed, the urethral flora was sampled by rolling the catheters onto culture plates. Cultures were obtained from 398 patients with sterile urine. Catheter surfaces were colonized less often in males than in females (16.8 and 67.0%, respectively) and yielded correspondingly fewer bacterial species per catheter. In both sexes, gram-positive species were isolated more frequently than gram-negative species (428 and 131 isolates, respectively). The bacteria most commonly isolated were Staphylococcus epidermidis and Streptococcus faecalis. Staphylococcus saprophyticus was not recovered despite the large proportion of young, sexually active women in the study population. Gram-negative bacteria, mainly Escherichia coli, gradually colonized the catheters over time but at all times were recovered in lower amounts the gram-positive organisms and less often in males than in females. Slime production by coagulase-negative staphylococci did not relate to the density of organisms recovered from the catheters or influence the presence of gram-negative bacteria. Despite frequent use of antibiotics in this population, the urethral flora remained highly susceptible to these drugs. Antibacterial activity was not detected on the catheters despite high concentrations in the urine. This suggests that systemic antibiotics have a very limited effect on resident urethral flora. Despite relatively small numbers in the urethra, gram-negative bacteria readily produce urinary tract infections in catheterized patients.