How the size of animal appendages are coordinated to scale with body size has been a longstanding question in developmental biology. We are using the Drosophila wing imaginal disc as a model to identify novel genes and mechanisms which finely tune this process. In the wing imaginal disc, Decapentaplegic (Dpp), a member of the Transforming Growth Factor β (TGF- β) superfamily of ligands and homologous to vertebrate Bone Morphogenic Protein (BMP) 2/4, signaling plays a crucial role in regulating organ size and patterning. In contrast, little is known about the related branch TGF-β/Activin pathway in regulating organ size. Here we show that myoglianin (myo), a TGF-β/Activin ligand, is required for proper growth of wing, haltere, and leg imaginal discs. The smaller imaginal discs of myo mutants phenocopies baboon, the TGF-β/Activin Type-I receptor, mutants. Using tissue specific RNAi we show that only Baboon-a isoform, but not Baboon-b or Baboon-c, is required for growth. Surprisingly, we also found that only the muscle derived Myoglianin, but not the glial derived, is required for proper growth of the wing disc. In summary, our work has identified a novel function of Myoglianin in wing disc growth control. More importantly this study highlights how the size of appendages like the insect wing can be modulated by structural organs like the body wall muscle using long range TGF-β ligands. In our future studies we will explore the intracellular molecular mechanism of Myo and Baboon signaling to determine whether they feed into the same pathways as the Dpp branch. .