Abstract: The invention features polypeptides that include an extracellular ActRII chimera. In some embodiments, a polypeptide of the invention includes an extracellular ActRII chimera fused to an Fc domain monomer or moiety. The invention also features pharmaceutical compositions and methods of using the polypeptides to treat diseases and conditions involving weakness and atrophy of muscles, bone damage, low red blood cell levels (e.g., anemia or blood loss), low platelet levels (e.g., thrombocytopenia), low neutrophil levels (e.g., neutropenia), fibrosis, metabolic disorders, and/or pulmonary hypertension.