Ecdysteroid and sequiterpenoids juvenile hormones play a gonadotrophic role in the insect adult female vitellogenesis. The molecular basis of hormone action has been analyzed in great detail in flies and moths, but rarely in primitive insect orders. The primitive hemimetabolous insect Periplaneta americana was used, as a model, to isolate and characterize, for the first time, two cDNAs of RXR/USP, a component of the heterodimeric ecdysone receptor. These two cDNAs correspond to two isoforms, named PamRXR-S (short form) and PamRXR-L (long form). Both are identical except for 25 amino acids deletion/insertion located in the loop between helices H1 and H3 of the ligand-binding domain. The two isoforms are differentially expressed in different tissues as revealed by RT-PCR and northern blot analysis. In fat body, brain, ovary, and muscle tissues, the predominant form was PamRXR-S, whereas PamRXR-L was abundant in ovaries. The PamRXR transcript was detected during all stages of vitellogenesis in the fat body with different levels. It was little low during the early vitellogenic period (days 2, 3), then a peak of increase was detected during days 4-6 (day 5) which was followed by another peak of increase at the end of vitellogenesis, day 9. We assumed that PamRXR might play a dual role of induction of vitellogenin through JH at early vitellogenesis and suppression through 20E during late vitellogenesis. The present work will pave the way for several other investigations to understand both the ecdysteroid-dependent genetic hierarchy and JH mechanism controlling vitellogenesis in the American cockroach, P. americana.