The laminin receptor alpha7beta1 is enriched at the myotendinous junctions, and mice with a targeted inactivation of the alpha7 gene develop a form of muscular dystrophy that primarily affects this structure. By ultrastructural analysis of alpha7-deficient mice, in comparison with wild-type and mdx mice, we attempted to elucidate the role of alpha7 integrin for the integrity and function of the myotendinous junctions. Ultrastructurally, myotendinous junctions of alpha7-deficient myofibers lose their interdigitations and the myofilaments retract from the sarcolemmal membrane, whereas the lateral side of the myofibers remains morphologically normal. The basement membrane at the myotendinous junctions in alpha7 -/- mice is significantly broadened, and immunogold-histochemistry has demonstrated that the laminin alpha2 chain is not localized here but, instead, in the matrix of the neighboring tendon. In contrast, mdx mice have normal myotendinous junctions, with a matrix protein pattern also found in wild-type mice, however the lateral sides of the myofibers are severely damaged. These results suggest that the alpha7beta1 integrin is a major receptor connecting the muscle cell to the tendon and helps to organize the myotendinous junction, whereas the dystrophin-glycoprotein complex is necessary for the lateral integrity of the muscle cell.