1. Vitamin B12 metabolism was studied in bats of the species Rousettus aegyptiacus, which live on an all-fruit diet in the wild. 2. There was a wide range in values for the serum vitamin B12 concentration of newly captured bats, but bats captured in the early spring had significantly higher mean serum vitamin B12 levels than bats captured in the early autumn. 3. There was an exponential decrease in serum vitamin B12 concentration with time in captivity for bats fed on a vitamin B12-deficient, all-fruit diet; the biological half-life was 80 d in serum, 109 d in liver and 164 d in kidney. 4. The main storage organ for vitamin B12 in the bats was the liver, mean content 1067 ng vitamin B12. After 50 d, injected [57Co]cyanocobalamin had equilibrated with body vitamin B12 stores, and 17% of the retained radioactivity was present in the liver. From these results it was calculated that the total body vitamin B12 content of the bat was c. 6500 ng. 5. The biological half-life of injected [57Co]cyanocobalamin was 70--88 d and the calculated daily requirement was 50--60 ng (0-2 mug/kg body-weight per d). 6. As serum vitamin B12 levels decreased, serum folate levels increased. The erythrocyte folate concentration increased significantly after 130 d on the all-fruit diet and then decreased to the initial values after 190 d. 7. Vitamin B12 metabolism in the fruit bat is similar in many respects to that of man, but on a 'weight-for-weight' basis the bat has a 5- to 15-fold greater requirement for this vitamin. 8. Vitamin B12 deficiency may be induced fairly rapidly in fruit bats fed on an all-fruit diet.