  Our high-time-resolution observations reveal that individual main pulses from
the Crab pulsar contain one or more short-lived microbursts. Both the energy
and duration of bursts measured above 1 GHz can vary dramatically in less than
a millisecond. These fluctuations are too rapid to be caused by propagation
through turbulence in the Crab Nebula or the interstellar medium; they must be
intrinsic to the radio emission process in the pulsar. The mean duration of a
burst varies with frequency as $\nu^{-2}$, significantly different from the
broadening caused by interstellar scattering. We compare the properties of the
bursts to some simple models of microstructure in the radio emission region.
