Variability has proven to be an essential aspect of the study of
 active galactic nuclei and has posed the strongest constraints on
 theoretical models. Here we propose to study the variability of the
 prototype BL Lac object PKS 2155-304 at several wavelength bands
 within the wide range covered by ISO in order to determine variability
 time-scales and the correlation of variability at different
 frequencies. A transition from optically thin to optically thick
 synchrotron emission is expected around about 30 micron. The
 correlation of light curves above and below this threshold should
 prove unambiguously whether this is or is not the case. PKS 2155-304
 is strongly and rapidly variable at all frequencies above the near
 IR while it is less strongly variable in the radio band. The proposing
 team has been involved in multifrequency campaigns, the last of which
 took place in May 1994. Multifrequency coverage simultaneous to the
 proposed ISO observations will be attempted. This proposal is
 complementary to the one submitted by the same team for a detailed
 spectral study of the source (OBSID: ATREVES, PROPID: PKS_SPEC).
 The variability study will set constraints on the size of the emitting
 region with important implications for other physical parameters, like
 the magnetic field and the beaming factor. This is one of the very few
 X-ray bright BL Lacs, for which this type of study can be performed
 due to its high brightness and variability at all energy bands.
 The ISO range is critical for distinguishing the physical properties
 of X-ray bright and radio-bright BL Lacs. If the ISO observations
 should proof that the flat radio to IR spectrum of PKS2155-304 is due
 to high selfabsorption, a small emission region with high magnetic
 field would be indicated which may represent a physical distinction
 with respect to the (expected) thin synchrotron emission in the ISO
 band from radio bright BL Lacs.