  We consider the conductivity $\sigma_{xx}$ of graphene with negligible
intervalley scattering at half filling. We derive the effective field theory,
which, for the case of a potential disorder, is a symplectic-class
$\sigma$-model including a topological term with $\theta=\pi$. As a
consequence, the system is at a quantum critical point with a universal value
of the conductivity of the order of $e^2/h$. When the effective time reversal
symmetry is broken, the symmetry class becomes unitary, and $\sigma_{xx}$
acquires the value characteristic for the quantum Hall transition.
