Insect molting and metamorphosis are regulated by ecdysteroids. The action of ecdysteroids is mediated via ecdysone receptor (EcR) and its heterodimer, ultraspiracle protein (USP). Both EcR and USP are members of the nuclear receptor superfamily and ligand dependent transcriptional factors. Transcriptional factors are often subjected to regulation by phosphorylation. Our recent studies in salivary glands of Drosophila melanogaster have shown that both EcR and USP are phosphoproteins and their phosphorylation is mediated by protein kinase C (PKC). Inhibition of PKC by PKC specific inhibitor blocked USP phosphorylation, leading to subcellular localization change of USP and inhibition of 20E-induced gene and protein expression. The composite data suggest that protein phosphorylation plays an important role in 20E-signaling pathway.