Ecdysteriods and juvenile hormone play critical role in regulation of insect growth and development. The most active form of ecdysteroids, 20-hydroxyecdysone (20E), transduces its signal through a heterodimeric complex of two nuclear receptors, the ecdysone receptor (EcR) and the ultraspiracle (USP), a homolog of the vertebrate retinoid X receptor (RXR). In the present study, we analyzed genome sequence information and identified two isoforms each of EcR and RXR in the red flour beetle, Tribolium castaneum. Developmental expression pattern of these two receptors was investigated using the real time quantitative RT-PCR. Injection of dsRNAs for EcR and RXR isoforms at different developmental stages resulted in blocking of metamorphosis. These results suggested that these two receptors play key roles in metamorphosis of Tribolium castaneum.