The morphogenetic effects of Precocene II in nymphs of 1st-,2nd-,3rd-, and 4th- instar of Rhodnius prolixus were studied. A high number of insects underwent precocious metamorphosis by topical treatment (10-30 µg/.insect) or adding the drug to the blood meal (25 µg/ml of blood).Adultoids of 4th- and 5th-instar, obtained by both treatments of 3rd- and 4 th-instar, respectively had mainly fully adult abdominal cuticle, ocelli, three segmented tarsi, rudimentary wings containing adult articulation with the thorax and deformed genitalia while adultiforms of 2nd- and 3rd-instar had very slight imaginal morphological characteristics. Topical treatment induced a high mortality in nymphs of 1st-instar and Precocene II given orally produced a high number of adultoids in this instar. The mode of application and the action of Precocene II on different nymphal instars were interpreted.