A total of 32 slender lorises (Loris tardigradus) kept in captivity were investigated postmortem. In five adult lorises (4 females, 1 male), the gallbladder contained either one gallstone or multiple gallstones. Except for one 5-yr-old female, the affected individuals were between 11 and 13 yr of age. All of the gallstones consisted of 100% cholesterol. Besides other predisposing factors such as species, sex, increasing age, and diabetes mellitus, nutrition may play a major role in the occurrence of these gallstones.