  The helicity dependence of the $\vec{\gamma} \vec{p} \to p \pi^0$ reaction
has been measured for the first time in the photon energy range from 550 to 790
MeV. The experiment, performed at the Mainz microtron MAMI, used a
4$\pi$-detector system, a circularly polarized, tagged photon beam, and a
longitudinally polarized frozen-spin target. These data are predominantly
sensitive to the $D_{13}(1520)$ resonance and are used to determine its
parameters.
