Mucinous carcinomas are very rare in the biliary tract. Only twenty cases of mucinous carcinomas of the gallbladder have been reported so far of which the pure mucinous (colloid) phenotype is exceedingly rare. We report one such rare case of a 55 year old lady with history of exposure to barium fumes in the past, who presented to our hospital with features of biliary obstruction. Radiological examination revealed neoplastic thickening of the gallbladder along with cholelithiasis and choledocholithiasis. Cholecystectomy was performed and the specimen showed thickened gallbladder wall with a glistening grey white infiltrative growth. Microscopy showed pure mucinous (colloid) adenocarcinoma, the entire tumor being composed of pools of extracellular mucin with clusters of tumor cells floating in them with no other component.