  Candidate astrophysical acceleration sites capable of producing the highest
energy cosmic rays (E > 10^{19.5} eV) appear to be at far greater distances
than is compatible with their being known particles. The properties of a new
particle which can account for observations are discussed and found to be
tightly constrained. In order to travel 100's or 1000's of Mpc through the
cosmic microwave background radiation without severe energy loss and yet
produce a shower in Earth's atmosphere which is consistent with observations,
it must be a hadron with mass of order a few GeV and lifetime greater than
about 1 week. A particle with the required properties was identified years ago
in the context of supersymmetric theories with a very light gluino. Laboratory
experiments do not exclude it, as is discussed briefly.
