  The Infrared Space Observatory (ISO) observed the BL Lac object PKS 2155-304
16 times from 1996, May 7 to June 8, with both the ISOCAM camera and the
ISOPHOT photometer, as part of a more general multiwavelength campaign. Two
additional observations were performed on 1996, November 23 and 1997, May 15.
This is the first time that there are simultaneous mid and far infrared data in
a multiwavelength monitoring of a BL Lac object. We obtained four light curves
at 4.0, 14.3, 60 and 90 microns, and a broad-band filter spectrum from 2.8 to
170 micron. No variability was detected in the infrared, although the source
was varying at shorter wavelengths. The IR spectrum can be fitted by a single
power law with an energy spectral index alfa = 0.40 +/- 0.06 and it can be
explained as due to synchrotron emission only, with no noticeable contributions
from thermal sources. Using the simultaneous data, we constructed the SED of
PKS 2155-304.
