Internal fistula is most common between gall bladder and -

Biliary fistula is a complication of gallstone (90% of Biliary fistulas are due to gallstone). The inflamed gall bladder becomes adherent to adjacent stomach, duodenum, colon and necrosis develops at the site leading to the development of fistulas. Most common site of fistula is duodenum 2nd most common site is colon. Ref : Hamilton Bailey, 10/e, p 432