We propose an  ISO target of Opportunity observation of the BL Lacartae
object Markarian 501 which is currently undergoing a major outburst at TeV
energies. According to many current theories of the emission processes in
blazras, the far-IR continuum provides the seed photons for inverse Compton
gamma rays, so a gamma-ray outburst should be accompanied (and preceeded) by a
far-IR outburst. Unfortunately, since the launch of ISO, only one blazar
outburst (3C279) has been observed by ISO, and that in PV phase where the
quality of the ISO data are less than optimum. Furthermore no Ultra High Energy
(UHE) gamma ray measurements were available. This may represent the only chance
with ISO to investigate any IR/UHE gamma ray linj in flaring blazars.
An accurate measurement of the IR continuum during the current outburst will
yield major diagnostics of the physical processes in the emission region of
these enigmatic objects.