A comparative study of the hepatic bile ducts of man and laboratory animals was made by means of scanning electron microscopy of biliary tract casts. In man, the large intrahepatic bile ducts at the hilum have many irregular side branches and pouches which are all situated in one plane corresponding anatomically to the transverse fissure (porta hepatis). At the bifurcation, some of the side branches from two or three bile ducts communicate with each other. The anastomosing plexus so formed provides communications between the main bile ducts. Bile ducts of the rhesus monkey show a similar but less well developed structure. In the pig, many pouches are observed around not only large but also small bile ducts. No such structures are observed in dog, guinea pig, or rabbit bile ducts. These irregular side branches and pouches correspond to the "vasa aberrantia" and "parietal sacculi" described by L. S. Beale (The Liver. Lecture on the Principles and Practice of Medicine, p 47. London, Churchill, 1889), and they may store and modify bile. The plexus described in man in this report may provide an anatomical basis for incomplete biliary obstruction without cholestasis. A role in biliary atresia and in Caroli's disease is also raised.