1. The esterases, acid and alkaline phosphatases. and glycogen were investigated in the antennal gland of the crayfish by histochemical procedures. Gel electrophoresis was employed to further characterize the esterases.2. A sexual dimorphism occurred in the labyrinth characterized by an intense reaction for the ali-esterases in the male contrasted to a weak reaction in the female. Eserine-sensitive esterases were also observed in the coelomosac, nephron tubule, and bladder. Both alkaline phosphatase and acid phosphatase were localized in the luminal border of the labyrinth. Acid phosphatase was also observed in the coelomosac and as granules in cells of the labyrinth. Glycogen was most concentrated in the coelomosac and bladder but was observed in the other areas of the gland and its lumen in less concentration.3. The significance of the above reactions was discussed in relation to their roles in the function of the antennal gland.