  In May 1994 the BL Lac object PKS 2155-304 was observed continuously for ~10
d with IUE and EUVE and for 2 d with ASCA, as well as with ROSAT and with
ground- based radio, IR, and optical telescopes. The light curves show a
well-defined X-ray flare (x2 increase in 1/2 d, with similar decay) followed by
a broader, lower amplitude EUV flare ~1 day later, and a broad, low-amplitude
UV flare ~2 d later (amplitude ~35%, duration >2 d). In the preceding week
there was at least one X-ray flare of comparable amplitude or perhaps ongoing
stochastic X-ray variations. An extremely rapid change in UV flux was seen at
the beginning of the IUE observation. Assuming the central X-ray, EUV, and UV
events are associated, the lags, the decrease of amplitude with wavelength, and
the broadening of the temporal profile with wavelength are all qualitatively as
expected for synchrotron emission from an inhomogeneous relativistic jet. We
can rule out a Fermi-type shock acceleration event or a pair cascade in a
homogeneous synchrotron-emitting region. A homogeneous region is still possible
if there was an instantaneous (t<<hours) injection of high energy electrons
that emit first at X-ray energies. Alternatively, the data are consistent with
a compression wave or other disturbance crossing a region with stratified
particle energy distributions (e.g., behind a shock front and/or in an
inhomogeneous jet). The present light curves are in sharp contrast to November
1991, when the amplitude was wavelength independent and the UV lagged the
X-rays by less than ~3 hours. This means that the origin of rapid
multiwavelength variability in this blazar is complex, involving at least two
different modes. (abbreviated abstract)
