Porosomes are inverted cup-shaped, presynaptic structures where secretory vesicles transiently fuse and expel their contents to the extracellular space. It is known that hair cells of hearing and balance secrete transmitter from synaptic vesicles during sensory signal transduction, but it was previously unknown whether these sensitive mechanosensory cells possess porosome structures that could participate in the secretory process. In this chapter, we provide evidence that porosome structures indeed exist in the hair cell, suggesting a mechanism of hair-cell transmitter secretion different from that of the exocytotic process currently proposed. We also discuss molecular mechanisms consistent with the porosome model.