The bacterial colonization of urethra and urine was studied over long periods in 16 hospitalized women with long-term indwelling bladder catheter. The cultured flora was polymicrobic and, except for Proteus mirabilis and Escherichia coli, rapidly changing. The colonization patterns showed marked inter-species variations. P. mirabilis was the species most commonly found, and in the urethra it was significantly more persistent than the other species. Unlike the other species, P. mirabilis was rarely found in urine without concomitant urethral growth. Prophylactic measures aimed to reduce the risk of permanent colonization by this pathogen, which is rendered particularly harmful by its urease production, should therefore be directed towards the urethra and the periurethral area.