The regulation of the soldier caste in termites involves a self-regulatory mechanism of negative feedback once a specific proportion is achieved. In this process, juvenile hormone (JH) plays an important role. Two groups of compounds with known JH regulatory functions in insects were tested for their effects on Coptotermes formosanus Shiraki soldier caste formation. One group was the antijuvenile hormone agents precocene I and precocene II. The other group was biogenic amines including dopamine, octopamine, serotonin and tyramine. Filter paper bioassays in Petri dishes showed that precocene I at 100 μg/dish significantly delayed the formation of the first presoldier and the first soldier. Additionally, precocene I significantly reduced the soldier proportion at 40 d after treatment as compared with controls. Precocene II had no effect. We also found no biogenic amine effect on termite soldier formation. These results may reflect the importance of precocene I in caste control and a reduced importance in biogenic amines to the synthesis or suppression of JH in termites.