ISO observations of the BL Lacertae object PKS 2155-304

The BL Lacertae object PKS 2155-304 was observed by the infrared Space Obse rvatory in May and June 1996, during a multiwavelength campaign. These are the first observations in the mid- and far-infrared bands since IRAS. In th e observing period, the source showed no detectable time variability at 4.0 , 14.3, 60 and 90 mu m. The spectrum from 2.8 to 100 mu m is well fitted by a single power law with energy spectral index alpha = 0.4, intermediate be tween the flatter radio spectrum and the steeper simultaneous optical spect rum. The overall infrared to X-ray spectral energy distribution can be well explained by optically thin synchrotron emission, with negligible contribu tions from thermal sources, We also show that the host galaxy flux is negli gible in this spectral range.