In current models of transforming growth factor‐β (TGF‐β) family signaling, type II receptors activate specific activin receptor‐like kinase (ALK) type I receptors. These serine/threonine kinases activate ligand‐dependent receptor regulated (R)‐Smad by phosphorylating carboxy‐terminal serines. We found that the receptor expression levels affected the phosphorylation and activation of the two R‐Smad subclasses, activin/TGF‐β‐specific (AR‐Smad) and bone morphogenetic protein (BMP)‐specific (BR‐Smad). Co‐expressing constitutively active type I and type II receptors in COS7 cells resulted in the phosphorylation of both R‐Smad subclasses in a ligand‐independent manner. This was verified using in vitro kinase assays. In untransfected B16 melanoma cells, TGF‐β1 and BMP‐2 induced phosphorylation of both R‐Smad subclasses, and TGF‐β1 up‐regulated the inhibitor of differentiation (Id) gene, which is usually regulated by BMP. By contrast, BMP‐2 up‐regulated plasminogen activator inhibitor‐1 (PAI‐1), which is an AR‐Smad‐regulated gene. Except for ALK4 and ALK6, levels of type I and type II receptor mRNAs were higher in B16 cells than in HeLa and HepG2 cells, in which TGF‐β1 and BMP‐2 induced phosphorylation of only the expected R‐Smad. These results help to explain the diverse effects of this ligand family.