Colony organization and cohesiveness is ultimately controlled by the functions carried out by the various castes. Caste development is largely in the control by juvenile hormone (JH) which in termites appears to be transferred among individuals and to act as a primer pheromone. Tritium-labeled (3H) JH was topically applied on Coptotermes formosanus workers and soldiers to monitor the transfer pathway of JH. JH passed through the cuticle gradually. However, the termite’s cuticle remains radioactive even after molting. We show that JH can be transferred from worker-to-worker, worker-to-soldier, soldier-to-worker, but not soldier-to-soldier. The mode of transfer appears not to be trophallaxis alone. Soldiers received JH from workers when the soldier to worker proportion was high, whereas JH worker-to-soldier transfer did not occur when soldier to worker proportion was low. Topically applied 3H-JH to workers caused a molt 9 days after treatment and newly-differentiated pre-soldiers carried 3H-JH mostly in their heads. 3H-JH was detected in the frontal gland secretion of the newly-molted soldiers. JH serves as a primer pheromone in termites to regulate caste differentiation, specifically in soldier caste differentiation. Soldiers play a key role in the distribution of JH throughout the colony.