  A measurement of the double-polarization observable $E$ for the reaction
$\gamma p\to \pi^0 p$ is reported. The data were taken with the CBELSA/TAPS
experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol
(C$_4$H$_9$OH) target, which provided longitudinally-polarized protons.
Circularly-polarized photons were produced via bremsstrahlung of
longitudinally-polarized electrons. The data cover the photon energy range from
$E_\gamma =600$~MeV to $E_\gamma =2310$~MeV and nearly the complete angular
range. The results are compared to and have been included in recent partial
wave analyses.
