Bacterial biofilms were observed on 69 of 75 catheters taken from patients undergoing long-term bladder management. Ten catheters were colonized by pure cultures of Proteus mirabilis. In each of these cases the bacteria formed layers on the catheter surface, underlying encrustations of struvite and hydroxyapatite which partially or completely occluded the catheter lumen. Encrustation was also apparent on catheters colonized by P. mirabilis plus other species, but was rarely seen on catheters colonized by non-urease-producing species. These observations support the hypothesis that catheter encrustation is brought about by the activity of urease-producing biofilms and confirms that the main target in the control of catheter encrustation should be P. mirabilis.