Mutations in the gene coding for the alpha 2 chain of laminin-2 and -4 (merosin) cause a severe form of congenital muscular dystrophy in humans and mice. To establish a defined model for in vitro and in vivo studies of the role of laminin alpha 2/merosin in development and cell and tissue function, we generated several lines of mutant embryonic stem (ES) cell with disruption of the laminin alpha 2 chain gene. We find that homozygous mutant ES cells differentiate normally in vitro, giving rise to cardiomyocytes, myotubes, and smooth muscle cells in addition to many other cell types. However, the myotubes that are formed are unstable. They detach, collapse, and degenerate, a process which is initiated at the appearance of the mature, contractile phenotype of the cells. We propose that the detachment and death of contracting myotubes in vitro has its counterpart in vivo and that contraction-induced myofiber damage, along with the lack of survival cues provided by laminin alpha 2/merosin, is a significant contribution to muscle degeneration in merosin-deficient muscular dystrophy.