The fish gallbladder, like the mammalian one, is an accessory organ of the digestive system that stores and secretes concentrated bile. This bile has several functions, such as facilitating several digestive functions, eliminating conjugated metabolites in the liver (including xenobiotics), and participating in the enterohepatic bile circulation. The quality of the bile itself and its precursors in fish have been surveyed and studied by Haslewood (1978). It was observed that diet plays a major role in determining the type of bile salts an animal has. Bile salts can have a deleterious action on the gallbladder wall, but this potential threat is balanced by the beneficial presence of bound cholesterol (Jacyna et al., 1986) as well as mucus. The main bile salts have been characterized in fishes according to systematic and dietary habits but only in a limited number of species (Haslewood, 1978; Cornelius, 1986). In consequence, it is difficult to discuss whether the fine structures of gallbladder and bile duct epithelium reflect the make-up or alteration of the bile composition due to changes in diet. One can just assume that, as in the mammalian gallbladder, the epithelium of the cystic duct and of the gallbladder in fish is able to concentrate the bile by altering its ionic and water permeability. Little is known about the regulation of bile acids and their impact on the liver and biliary tract ultrastructure and metabolism resulting from xenobiotics. The same is true of the gallbladder. The general microscopic anatomy and the production of mucinous compounds by the surface epithelium are some features common to the fish and the mammalian gallbladder (Gilloteaux et al., 1992, 1993a-c; Karkare and Gilloteaux, 1995; Karkare et al., 1995).