In lower termites, the soldier is the only sterile caste. Through hormonal control, soldiers seem to promote caste differentiation of workers to the reproductive caste. However, little data exist on how hormonal levels change in soldiers and workers in the field and how this may tie into the seasonal reproductive formation. Our objective was to collect baseline data on hormonal changes in colonies of subterranean termites in the field. Over a 1-yr period, the seasonal variation of juvenile hormone III (JH) titers was determined for Coptotermes formosanus Shiraki soldiers and workers collected monthly from field colonies infesting isolated cypress trees (Taxodium distichum L. Rich.). A total of five colonies were evaluated. Soldier JH titers leveled from February through May, with peak values in June and July. JH titers in workers followed a similar trend. Both soldier and worker JH titers remained at low levels in the fall and winter. This is the first effort to correlate physiological and hormonal aspects with soldier production in the field. Seasonal changes in JH titers as an intrinsic mechanism for reproductive caste differentiation is also discussed.