  The anti-forcing number of a perfect matching $M$ of a graph $G$ is the
minimal number of edges not in $M$ whose removal make $M$ as a unique perfect
matching of the resulting graph. The anti-forcing spectrum of $G$ is the set of
anti-forcing numbers of all perfect matchings of $G$. In this paper we prove
that the anti-forcing spectrum of any cata-condensed hexagonal system is
continuous, that is, it is an integer interval.
