  We study resistor diode percolation at the transition from the
non-percolating to the directed percolating phase. We derive a field theoretic
Hamiltonian which describes not only geometric aspects of directed percolation
clusters but also their electric transport properties. By employing
renormalization group methods we determine the average two-port resistance of
critical clusters, which is governed by a resistance exponent $\phi$. We
calculate $\phi$ to two-loop order.
