Biofilms were present on 16 of 33 urethral catheters examined. In 11 cases the catheter carried a different microbial flora from that of the bladder urine. The length of time the catheter was in situ did not influence biofilm formation, and all types of materials tested supported biofilm growth. Biofilms were seen on 2 of the 7 catheters where prophylactic antibiotics had been used.