The vibrations of an artificially blown clarinet were studied by a photoelectric method in which light from a source placed at the bell of the instrument passed through the aperture between the reed and the mouthpiece and into a photomultiplier tube whose output was observed on an oscilloscope. It was found that for loud tones the aperture is practically completely closed for about one-half cycle and completely open for the other half. For softer tones the aperture does not close completely and the reed motion becomes nearly sinusoidal. A small condenser microphone inserted in the clarinet mouthpiece allowed simultaneous observations of the reed vibrations and air column vibrations to be made. It was found that during that part of its motion that the reed is not in contact with the mouthpiece it follows quitef aithfully the variations in mouthpiece air pressure. The sound pressure level in the mouthpiece reaches 166 db for loud tones; this value checks well with the pressure required for blowing the instrument. The increased loudness of the clarinet on harder blowing is due primarily to the increased production of harmonics.