During 1981 we operated, with the cooperation of the U. S. Geological Survey, a 9 station digital seismic array on Mount St. Helens volcano in Washington State. One of our stations was placed inside the crater of the volcano, six were located on the flanks of the volcano within two km of the crater and two were approximately ten km from the crater. Four of the instruments recorded three components of motion and the remaining five recorded only the vertical component. In addition to the monitoring and recording performed by the digital instruments, we carried out a one day experiment during which we complimented our crater monitoring seismometer by the addition of two ink recording instruments. During the one day experiment we had six observers, including 4 USGS scientists who had significant experience working and making observations in the crater, recording times of rockfall, felt earthquake occurrence and changes in steam emissions from the dome in the center. Using information obtained during the one day experiment we are able to classify seismic events recorded by our digital instruments as earthquakes, rockfalls, helicopter noise and a type of event that is unique to volcanoes which we call long period. Wave-forms of these long period events have a duration of up to 30 seconds and a spectrum that is peaked at approximately 2 Hz. The frequency at which the peak in the spectrum occurs is nearly the same at all stations which means that the unique waveform of long period events is due to a source effect, not a path effect. The peak frequency is fairly insensitive to the amplitude of the signal which means that the size of the source region is constant, independent of the signal amplitude. Long period events were not felt and were accompanied by no visible changes inside the crater which lead us to conclude that they are some sort of seismic disturbance generated inside the volcano.