The role of the vocal tract in clarinet performance has been investigated by examining the vocal tract impedance, Z[inf u]. Theory predicts that Z[inf u] peaks aligned with harmonics of the pitch frequency stabilize the oscillation. The vocal tract impedance was measured directly with a one-microphone technique and indirectly from a calculation using the continuity of flow equation p[inf u]/Z[inf u]+p[inf d]/Z[inf d]=0, where p[inf u] and p[inf d] are the mouth and mouthpiece pressures and Z[inf d] is the instrument impedance. The continuity equation was verified for single tones and then used to examine the role of the vocal tract in a variety of musical situations. In orchestral excerpts, the performer tends to align resonances with the first and/or second harmonics, and there is some dependence on the musical context. For multiphonics, the performer creates a resonance that supports an oscillation at a linear combination of the audible pitch frequencies. For tones played with pitchbend, induced by slacking the jaw muscles, the playing frequency is controlled by the frequency of a large-amplitude vocal tract resonance, for which |Z[inf u]|(approximately equal to)100--400 CGS ohms. Z[inf u] changes only slightly when clarion tones are played without the register key, suggesting that other variables are involved.