The efficacy of two ActRIIB ligand‐trapping agents (RAP‐031 and RAP‐435) in treating muscular dystrophy was examined by determining their morphological effects on the severely dystrophic triangularis sterni (TS) muscle of the mdx mouse, a model for Duchenne muscular dystrophy. These agents trap all endogenous ligands to the ActRIIB receptor and thereby block myostatin signaling in a highly selective manner. Short‐term (1 month) and long‐term (3 months) in vivo treatment of 1‐month‐old mdx mice increased myonuclei and fiber cross section (FCS) density but did not alter individual fiber size. Vehicle‐treated mdx mice exhibited age‐dependent increases in myonuclei and FCS density, and age‐dependent reductions in centronucleation that were each enhanced by treatment with RAP‐435. Distributions of FCS area (FCSA) in the mdx TS were 90% identical to those from untreated age‐matched nondystrophic mice and were unaltered by the substantial fiber hyperplasia observed with age and RAP‐435 treatment. These results were inconsistent with injury‐induced fiber regeneration which produces altered FCSA distributions characterized by a distinct class of smaller regenerated fibers. Nondystrophic mice exhibited a constant postnatal density of fiber cross sections and myonuclei, and RAP‐435 treatment of nondystrophic mice increased TS mean FCSA but had no effects on myonuclei or FCS density. These results demonstrating a continual postnatal proliferation and fusion of satellite cells and a response to myostatin blockade characteristic of developing prenatal muscle suggest that the lack of dystrophin directly results in unrestrained postnatal satellite cell activation that is not necessarily dependent upon prior fiber degeneration. J. Cell. Physiol. 232: 1774–1793, 2017. © 2016 Wiley Periodicals, Inc. We examined the efficacy of two ActRIIB ligand‐trapping agents in altering the morphology of a severely dystrophic muscle in the mdx mouse. The results indicate that severely dystrophic muscle exhibits monotonic postnatal increases in myonuclei and fiber density that are not associated with alterations in the distribution of fiber cross‐sectional areas. Long‐term treatment with an ActRIIB ligand‐trapping agent substantially enhanced this postnatal hyperplasia and reduced centronucleation. These results demonstrating a continual postnatal proliferation and fusion of satellite cells and a response to myostatin blockade characteristic of developing prenatal muscle suggest that the lack of dystrophin directly results in unrestrained postnatal satellite cell activation that is not necessarily dependent upon prior fiber degeneration.