Neurally released agrin is thought to cluster acetylcholine receptors (AChRs) and other synaptic proteins in the postsynaptic membrane during synaptogenesis at the neuromuscular junction. We have examined the binding of nerve and muscle agrins, which have dramatically different abilities to cluster AChRs, to the membrane proteins of Torpedo electric organ and C2 myotubes. Both bound with approximately nanomolar affinity to a single component identified as alpha-dystroglycan: agrin binding was blocked by antibodies to alpha-dystroglycan, and agrin bound to purified alpha-dystroglycan. Dystroglycan was altered in two genetic variants of C2 muscle cells that fail to form spontaneous clusters of AChRs and that show a diminished response to agrin. Antibodies that blocked alpha-dystroglycan binding, however, failed to block the clustering of AChRs by neural agrin. Although alpha-dystroglycan is the major agrin-binding protein in Torpedo and myotube membranes, its physiological role is unclear.