Urine samples from 19 nursing home patients with long-term urinary catheters were cultured every 3 months for 18 months. Providencia stuartii, present in 74% of the elderly and in 59% of urine specimens, was the most frequently isolated bacteria. The persistence of P. stuartii was significantly higher among females than among males. In order to study the epidemiology of bacteriuria in this nursing home, bacteria were characterized by biochemical tests, antibiotic susceptibility pattern, and restriction fragment length polymorphism (RFLP) analysis. The antibiotic susceptibility pattern indicated that each patient had two to three different strains of P. stuartii during the 18 months of follow-up. In contrast, the RFLP analysis revealed that a specific strain had persisted in the urinary tract of the patient during the entire follow-up period. According to the biochemical profile, 74% of the patients had the same bacteria in urine cultures, pointing to a common source of transmission. RFLP analysis, however, demonstrated different patterns of RFLP, suggesting concomitant multiple sources of infection.