The excitatory postsynaptic currents (EPSCs) recorded from eighth-nerve afferent terminals include many large signals thought to represent the simultaneous release of several vesicles. We performed paired whole-cell recordings of the hair cell and its afferent fiber in the auditory amphibian papilla of the adult bullfrog. The amplitudes of spontaneous EPSCs had a broad, Gaussian distribution with a mean of about -130 pA. The largest signals, with amplitudes exceeding -200 pA, displayed kinetics similar to those only a quarter as large, suggesting that the former stem from the highly synchronized release of multiple vesicles. Holding the hair cells at -80 mV eliminated the large spontaneous EPSCs, leaving a narrow amplitude distribution with a mean of -55 pA; the inclusion of 0.4 mM Cd²⁺ in the bath solution to block Ca²⁺ channels had a similar effect. During voltage-evoked exocytosis, we compared the synaptic charge transfer into an afferent terminal with the simultaneous increase in membrane capacitance of a hair cell. The resultant regression line has a linear slope that can used to calculate a single-vesicle capacitance of 50 aF, a value in close agreement with the electron-microscopic estimate of 45 aF. These results strongly support the hypothesis that the large spontaneous EPSCs are multiquantal and due to the synchronous release of several vesicles from a single synaptic ribbon active zone. We propose that these large events originate via a compound or coordinated mechanism of multivesicular release. Their physiological significance will be discussed.