We have developed a method for displaying the spectral structure of Jupiter's decametric radio S bursts on timescales down to a few microseconds, 2 orders of magnitude finer than has been achieved elsewhere. By employing an extremely sensitive antenna (640 dipoles, at 26 MHz) and selecting only relatively weak S bursts that possess the simplest possible spectral shape, we identify frequently occurring structural features that we associate with localized emission centers. On timescales having better than about 30 &mgr;s resolution we find that the S burst baseband oscillation (and therefore the RF oscillation) is modulated to form distinct pulses, which we refer to as subpulses. Still finer time resolution reveals that within individual subpulses the baseband (and RF) oscillation often displays segments in which the usually drifting phase term abruptly becomes essentially constant and, after remaining so for 10 to 100 &mgr;s, abruptly resumes its random drift. It is these abruptly starting and ending segments of phase coherence that we attribute to isolated powerful centers of cyclotron maser wave amplification, which happen for brief intervals to be the only ones that are active. We believe that the more usual phase-incoherent condition (i.e., one in which the instantaneous frequency drifts randomly within the emission band) is due to the fact that the resultant radiation is the sum of two or more components from neighboring wave amplification centers emitting at slightly different frequencies, with independently varying intensities. Possible models for the production of subpulses and phase coherent intervals are discussed. ¿ 1999 American Geophysical Union