A simple method is presented to construct the hierarchy of a negative
order equations from a given eigenvalue problem. The hierarchy of the
negative order equation we obtained is the natural continuation of a c
orresponding hierarchy of a higher order equation, although most of th
e negative order equations are complicated; we give their Lax represen
tations. Three examples are discussed to illustrate the effectiveness
of our method. (C) 1995 American Institute of Physics.