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The transforming growth factor-β (TGF-β) superfamily encodes a large group of proteins, including TGF-β isoforms, bone morphogenetic proteins and activins that act through conserved cell-surface receptors and signaling co-receptors. TGF-β signaling in insects controls physiological events, including growth, development, diapause, caste determination and metamorphosis. In this study, we used the red flour beetle, <i>Tribolium castaneum,</i> as a model species to investigate the role of the type I TGF-β receptor, <i>saxophone</i> (<i>Sax</i>), in mediating development. Developmental and tissue-specific expression profiles indicated <i>Sax</i> is constitutively expressed during development with lower expression in 19- and 20-day (6th instar) larvae. RNAi knockdown of <i>Sax</i> in 19-day larvae prolonged developmental duration from larvae to pupae and significantly decreased pupation and adult eclosion in a dose-dependent manner. At 50 ng dsSax/larva, Sax knockdown led to an 84.4% pupation rate and 46.3% adult emergence rate. At 100 ng and 200 ng dsSax/larva, pupation was down to 75.6% and 50%, respectively, with 0% adult emergence following treatments with both doses. These phenotypes were similar to those following knockdowns of 20-hydroxyecdysone (20E) receptor genes, <i>ecdysone receptor</i> (<i>EcR</i>) or <i>ultraspiracle</i>  <i>protein</i> (<i>USP</i>). Expression of 20E biosynthesis genes <i>disembodied</i> and <i>spookier</i>, 20E receptor genes <i>EcR</i> and <i>USP</i>, and 20E downstream genes <i>BrC</i> and <i>E75</i>, were suppressed after the <i>Sax</i> knockdown. Topical application of 20E on larvae treated with dsSax partially rescued the dsSax-driven defects. We can infer that the TGF-β receptor gene <i>Sax</i> influences larval-pupal-adult development via 20E signaling in <i>T. castaneum</i>.