Precocene II is a botanically derived chemical that inhibits the production of juvenile hormone (JH) in insects. The effects of this anti-JH compound on molting and growth by Rhodnius prolixus and Triatoma dimidiata and reproduction in R. prolixus were tested and the efficacy of precocene II as a fumigant was assessed. Precocene II induced precocious metamorphosis in both species when applied by either contact exposure or fumigation, and this effect could be prevented by juvenile hormone replacement therapy. The dosage effective in inducing precocious metamorphosis in T. dimidiata was similar to the EC50 previously reported for R. prolixus. The morphology of precocious adultoid T. dimidiata of different instars was similar to Rhodnius adultoids of corresponding instars. However, T. dimidiata was more sensitive than Rhodnius to the molt-inhibiting effects of precocene II. Rhodnius nymphs were fully susceptible to the anti-JH action of precocene when exposed for 24 hours one week before feeding. Precocene II was highly toxic to adult female Rhodnius and treatment of newly emerged females prevented oogenesis.