The interaction of coxsackievirus A13 with differentiating muscle cells, cultured from tissues of the fetal mouse, was studied. Cultures infected at that stage of myogenic differentiation characterized by the rapid formation of multinucleated myotubes produced maximum virus titers of over 107 plaque-forming units. Virus-induced cytopathic effect was characterized by a marked diminution in the number of multinucleated cells. The susceptibility of these cultures decreased appreciably when infection was initiated after the majority of the myotubes had formed. The demonstration of newly synthesized A13 virus antigen by immunofluorescence provided direct evidence that A13 virus replication occurred both in myoblasts and myotubes. The synthesis of A13 virus was markedly depressed in muscle cultures in which the formation of multinucleated cells was inhibited by BUDR or by fusion-inhibiting media. After reversal of this inhibition, the cultures acquired the increased susceptibility to A13 virus characteristic of cells undergoing myogenic differentiation. In contrast to the results obtained with coxsackievirus A13, the primary fetal mouse muscle cultures were resistant to poliovirus T1. It is suggested that changes in the surfaces of developing muscle cells may coincide with the formation and disappearance of specific virus receptors and thereby regulate the cell susceptibility to coxsackievirus A13.