The role of juvenile hormone on caste determination was investigated in the ant, Pheidole fervida SMITH, whose colony is comprised of three castes, worker, soldier and queen. The activities of the corpora allata (CA) of the three castes were evaluated by measuring the CA volume through the 3rd (last) instar and prepupal stage. Among the three castes, the volume of CA was largest in the queen, intermediate in the soldier, and smallest in the worker. Various doses of juvenile hormone analogue (JHA) were applied topically to the larvae, eggs and nursing adults. The results suggest that soldier determination occurs at least in the 3rd larval instar by JHA treatment and that the queen may be determined in the same way.