  We report on the discovery of a source which exhibits over 300% amplitude
changes in radio flux density on the period of hours. This source, J1819+3845,
is the most extremely variable extragalactic source known in the radio sky. We
believe these properties are due to interstellar scintillation, and show that
the source must emit at least 55% of its flux density within a radius of <16
microarcseconds at 5GHz. The apparent brightness temperature is > 5.10^{12} K,
and the source may be explained by a relativistically moving source with a
Doppler factor ~15. The scattering occurs predominantly in material only a few
tens of parsecs from the earth, which explains its unusually rapid variability.
If the source PKS 0405-385 (Kedziora-Chudczer et al 1997) is similarly affected
by local scattering material, Doppler factors of ~1000 are not required to
explain this source. The discovery of a second source whose properties are well
modeled by interstellar scintillation strengthens the argument for this as the
cause for much of the variations seen in intra-day variables (IDV).
