The biliary tract of cats is known to be free of autochthonous bacteria above the sphincter of Oddi. In this experiment we investigated whether transient bacterobilia occurs in the biliary system under normal conditions. Polyethylene tubes and human cholesterol stones were implanted surgically into the gallbladder of cats. Sham cholecystostomy was performed as control operation. These cats were euthanized at two, six, and 12 weeks, and the implants were removed, cultured, and studied by scanning electron microscopy (SEM). Cultures and SEM also were undertaken for material scraped from the mucosal surface of the biliary tract from these animals. Colonization of bacteria on the polyethylene tubes and the gallstones was found six and 12 weeks after implantation. Adherent bacterial biofilms were demonstrated on the surfaces of these implants. This experiment showed that transient bacterobilia exists in the feline biliary tract. The foreign body implants have facilitated the adhesion of planktonic bacteria in the bile onto their surfaces and have initiated the formation of adherent biofilms within which these bacteria persisted until the system was sampled.