We overexpressed a constitutively active form of the neuregulin receptor ErbB2 (CAErbB2) in skeletal muscle fibers in vivo and in vitro by tetracycline-inducible expression. Surprisingly, CAErbB2 expression during embryonic development was lethal and impaired synaptogenesis yielding a phenotype with loss of synaptic contacts, extensive axonal sprouting, and diffuse distribution of acetylcholine receptor (AChR) transcripts, reminiscent of agrin-deficient mice. CAErbB2 expression in cultured myotubes inhibited the formation and maintenance of agrin-induced AChR clusters, suggesting a muscle- and not a nerve-origin for the defect in CAErbB2-expressing mice. Levels of tyrosine phosphorylated MuSK, the signaling component of the agrin receptor, were similar, while tyrosine phosphorylation of AChRbeta subunits was dramatically reduced in CAErbB2-expressing embryos relative to controls. Thus, a gain-of-function manipulation of ErbB2 signaling pathways renders an agrin-deficient-like phenotype that uncouples MuSK and AChR tyrosine phosphorylation.