An optical laser differential interferometer, based on a modified differential interference contrast microscope, has been developed to measure the thermal motion of microscopic protrusions (stereocilia) that are the sites of mechanoelectrical transduction in auditory hair cells. The measurement sensitivity was limited athigh frequencies mainly by shot noise, at intermediate frequencies by acoustic interference, and at lowfrequencies by thermal drift. The power spectral density of the instrumental noise was found to be as low as 1.pm/VHz in the shot noise regime. We could, thus, measure the Brownian motion of hair bundles over thefrequency range from 1 Hz-100 kHz. Experimental data that test and demonstrate the sensitivity and spatialdiscrimination of the instrument were found to be in agreement with theoretical estimates. © 1990 Optical Society of America.