A small condenser microphone inserted into the mouthpiece of a clarinet enables the vibrations of the air column to be observed. At low intensities the pressure variation in the mouthpiece is practically sinusoidal; at higher intensities harmonics increase in amplitude, but the fundamental still predominates. The sound pressure level inside the mouthpiece for loud tones reaches 166 db; this checks well with the pressure required for blowing the instrument. Simultaneous observations of mouthpiece pressure and reed displacement show that, during that part of its motion that it is not in contact with the mouthpiece, the reed follows quite faithfully the pressure variations. The reed itself is primarily stiffness controlled; however, the damping of the reed motion by the player's lip is a very important factor in the performance of the instrument. The influence of the reed on the pitch of the tone produced by the instrument will be discussed.