<p>The retinoblastoma tumor suppressor pRB is required for skeletal myogenesis but its role in maintenance of post-mitotic skeletal muscle and the contribution, if any, of its relatives, p107 and p130, are largely unknown. Here, we show that targeted deletion of murine <i>Rb</i> in proliferating myoblasts during myogenesis, using a Pax7-Cre deleter line, leads to muscle fiber degeneration, short myotubes with elongated, large nuclei, reduced late muscle marker expression, and fetal death. These defects are recapitulated in primary myoblasts derived from Pax7-Cre:Rb^f/f mice, can be ameliorated by inhibition of autophagy in vitro, and are exacerbated in Pax7-Cre:Rb^f/f:p107^–/– but not Pax7-Cre:Rb^f/f:p130^–/– double mutant fetuses. In contrast, deletion of <i>Rb</i> on a wildtype or p107^–/– background in post-mitotic muscle, via an Mlc_<i>1f</i>-Cre deleter line, has no apparent impact on skeletal muscle homeostasis. However, approximately 10% of Mlc_<i>1f</i>-Cre:Rb^f/f:p130^–/– mice, with combined deletion of <i>Rb</i> and <i>p130</i>, exhibit reduced size, wobbly, waddling gait, along with muscle degeneration and dramatic reduction in skeletal muscle mass. The remaining Mlc_<i>1f</i>-Cre:Rb^f/f:p130^–/– mice had near normal posture and muscle mass, but certain muscle areas show extensive central nuclei, while whole muscles express elevated levels of Pax7 and autophagic markers, suggestive of excessive muscle degeneration and regeneration. These mice also display muscle fiber type redistribution accompanied by reduced PGC-1α expression. Thus, continuous pRB and p130 expression is required to maintain skeletal muscle homeostasis and prevent adult muscle degeneration. Moreover, genetic modifiers—yet to be defined—affect the balance between muscle atrophy and regeneration.</p>