  The mid infrared polarisation properties of four Ultraluminous Infrared
Galaxies (ULIRGs) have been investigated by broad band filter observations with
the ISOCAM instrument on board the Infrared Space Observatory (ISO). The
wavelength region from 5 to 18 mic. was selected where the emission from the
putative torus peaks. We report detection of polarisation for all ULIRGs
studied. The fractional polarisation ranges from about 3% up to 8%. The highest
polarisation is recorded in Mrk231 which has a clear AGN signature, whereas the
lowest is for Arp220, which is generally thought to be powered predominantly by
star formation. We discuss the various mechanisms that could give rise to the
polarisation and conclude that the most likely interpretation is that it is due
to magnetically aligned elongated dust grains. This is the same mechanism
believed to be operating in a number of galactic sources. The position angle of
polarisation could give the projected magnetic field direction and therefore
constrain models for the formation of the tori.
