  We present x-ray observations of the nearby BL Lac PKS 2155-304 taken when it
was undergoing a GeV/TeV gamma-ray outburst. During the outburst we measured
x-ray fluxes in the 2-10 keV band that are the largest ever observed for PKS
2155-304. Comparison of these November 1997 measurements and other x-ray
observations made contemporaneously with GeV or TeV gamma-ray observations
indicate that x-ray and gamma-ray emissions are correlated. Measurements with
x-ray all-sky monitors such as the ASM/RXTE and MOXE can therefore signal the
presence of outbursts at gamma-ray energies from PKS 2155-304.
