The associations of diet compositions with mucin secretion in gallbladder have not been investigated in dogs. This study aimed to examine the effects of a low-carbohydrate diet (LC) and a low-fat diet (LF) on bile mucin concentration and composition of gallbladder bile in six clinically healthy beagle dogs. After feeding of both diets, the bile mucin concentration was significantly decreased. In addition, there were significant decreases in the concentrations of taurochenodeoxycholic acid in bile, which is considered to promote mucin secretion, after feeding of both diets. The present study suggested that the proportions of carbohydrate and fat in diet affect the composition of gallbladder bile in dogs.