  Using the nonperturbative Schwinger-Dyson equation, we show that chiral
symmetry is dynamically broken in QED at weak couplings when an external
magnetic field is present, and that chiral symmetry is restored at temperatures
above $T_c \simeq \alpha\pi^2/\sqrt{2 \pi |eH|}$, where $\alpha$ is the fine
structure constant and $H$ is the magnetic field strength.
