We propose to obtain ISOCAM and ISOPHOT observations of the two
 reddest galaxies [(I-K)~6.5] which are currently known.  Hu and
 Ridgway, who discovered these objects in the field of the z=3.790
 quasar PC 1643+4631A, suggested, on the basis of their colors,
 that these were early type galaxies at z=2 to 3.  However, more
 recently, Frayer et al. have reported a detection of CO emission
 at the redshift 3.137 of a damped Ly alpha system lying towards
 the quasar and suggested that the ultrared objects may be reddened
 active or starbursting galaxies associated with this emission.
 This interpretation would imply that these sources should be
 strong 100-200 micron sources and easily detectable with ISO.
 Multiwavelength observations with ISO should determine the nature
 of these very unusual objects.