  We present a KAM theorem for presymplectic dynamical systems. The theorem has
a " a posteriori " format. We show that given a Diophantine frequency $\omega$
and a family of presymplectic mappings, if we find an embedded torus which is
approximately invariant with rotation $\omega$ such that the torus and the
family of mappings satisfy some explicit non-degeneracy condition, then we can
find an embedded torus and a value of the parameter close to to the original
ones so that the torus is invariant under the map associated to the value of
the parameter. Furthermore, we show that the dimension of the parameter space
is reduced if we assume that the systems are exact.
