  Daily monitoring of PKS 2155-304 with the IUE satellite throughout November
1991 has revealed dramatic, large-amplitude, rapid variations in the
ultraviolet flux of this BL Lac object. Many smaller, rapid flares are
superimposed on a general doubling of the intensity. During the five-day period
when sampling was roughly continuous, the rapid flaring had an apparent
quasi-periodic nature, with peaks repeating every ~0.7 days. The short- and
long-wavelength ultraviolet light curves are well correlated with each other,
and with the optical light curve deduced from the Fine Error Sensor (FES) on
IUE. The formal lag is zero but the cross-correlation if asymmetric in the
sense that the shorter wavelength emission leads the longer. The ultraviolet
spectral shape varies a small but significant amount. The correlation between
spectral shape and intensity is complicated; an increase in intensity is
associated with spectral hardening, but lags behind the spectral change by ~1
day. The sign of the correlation is consistent with the nonthermal acceleration
processes expected in relativistic plasmas, so that the present results are
consistent with relativistic jet models, which can also account for
quasi-periodic flaring. In contrast, currently proposed accretion disk models
are stronly ruled out by the simultaneous optical and ultraviolet variability.
