  We present a perturbative result for the temporal evolution of the fidelity
of the quantum kicked rotor, i.e. the overlap of the same initial state evolved
with two slightly different kicking strengths, for kicking periods close to a
principal quantum resonance. Based on a pendulum approximation we describe the
fidelity for rotational orbits in the pseudo-classical phase space of a
corresponding classical map. Our results are compared to numerical simulations
indicating the range of applicability of our analytical approximation.
