Myosin-1c (also known as myosin-Ibeta) has been proposed to mediate the slow component of adaptation by hair cells, the sensory cells of the inner ear. To test this hypothesis, we mutated tyrosine-61 of myosin-1c to glycine, conferring susceptibility to inhibition by N(6)-modified ADP analogs. We expressed the mutant myosin-1c in utricular hair cells of transgenic mice, delivered an ADP analog through a whole-cell recording pipette, and found that the analog rapidly blocked adaptation to positive and negative deflections in transgenic cells but not in wild-type cells. The speed and specificity of inhibition suggests that myosin-1c participates in adaptation in hair cells.