Electrophysiological evidence is presented that at least 30 percent of sartorius muscle fibres of adult frogs are innervated by two or more axons at a single end-plate zone. In these fibres, increasing stimulation of the common sartorius nerve led to the appearance of two or more distinct levels of end-plate potentials (e.p.p.) (or currents, measured by the voltage clamp technique). They had an identical time course, reversal potential and delay to nerve stimulation. When the recording microelectrode was moved along the same fibre and reinserted, both components of e.p.p. decreased proportionally. This indicated that both components of e.p.p. originated very closely to each other on the muscle fibre, presumably in one end-plate zone. Many fibres of the sartorius muscle of adult frogs therefore possess polyneural innervation of a single end-plate zone, which is otherwise typical for early stages of ontogenesis.