1. Carbohydrate levels in tissues of Aplysia californica were measured. In animals maintained in the laboratory without feeding for 1-2 weeks, the penis contained the highest concentration of glycogen. 2. Blood glucose ranged between 2.5 and 4.8 mg/dl and averaged 3.2 +/- 0.2 mg/dl. 3. Fed animals had significantly greater tissue glycogen levels than starved animals. The penis exhibited the smallest per cent increase in glycogen concentration in response to feeding. 4. In contrast, the hepatopancreas exhibited a large increase, attaining glycogen concentrations near that of the penis. 5. Most of the increase in total tissue glycogen in fed animals occurred in the hepatopancreas. 6. Starved and fed animals did not differ significantly in blood glucose concentration, thus indicating that blood glucose is regulated and excess glucose is stored. 7. The hepatopancreas is the major mobile glucose store. 8. The high concentrations of glycogen found in the penis were not affected by mating activity. 9. However, penis glycogen concentration increased greatly with reproductive maturation of Aplysia; in contrast, hepatopancreas glycogen concentration was relatively unaffected by development. 10. A structural role by carbohydrates in the penis is suggested.