  We investigate the proposal that primary cosmic rays of energy above the
Greisen-Zatsepin-Kuzmin cutoff are exotic massive strongly interacting
particles (uhecrons). We study the properties of air showers produced by
uhecrons and find that masses in excess of about 50 GeV are inconsistent with
the highest energy event observed. We also estimate that with sufficient
statistics a uhecron of mass as low as 10 GeV may be distinguished from a
proton.
