The Los Alamos plasmaspheric drift radio interferometer is a ground-based array that regularly measures periodic disturbances in the plasmasphere. These plasmaspheric density structures have been shown to depend on geomagnetic activity, as indicated by Kp. However, a direct storm time analysis of their behavior has not been done. This paper studies the amplitude, drift velocity, and location of these structures before, during, and after the onset of major geomagnetic storms. Distinct large-amplitude, storm time signatures are found during the first night after onset, continuing through the third night; there were significantly more storm time signatures during nighttime than daytime. The L shells on which the disturbances existed were found to decrease after storm onset, indicating a possible shrinking of the plasmasphere.¿ 1997 American Geophysical Union