An equistable graph is a graph for which the incidence vectors of the
maximal stable sets are the 0-1 solutions of a linear equation. A nece
ssary condition and a sufficient condition for equistability are given
. They are used to characterize the equistability of various classes o
f perfect graphs, outerplanar graphs, and pseudothreshold graphs. Some
classes of equistable graphs are shown to be closed under graph subst
itution. (C) 1994 John Wiley & Sons, Inc.