A two-beam interferometric technique is used to measure the fluctuations in the flying height of an air bearing slider above a quartz disk. Air bearing variations of less than 0. 5 nm are measured from 0 to 100 kHz. Mode excitation is only from disk runout and other sources natural to the rigid disk recording environment. The dynamic behavior of the leading and trailing edges of a slider is shown. Both suspension and air bearing resonant modes are observed at the slider and their frequencies measured.