  We investigate the dynamical impact of cosmic rays in cosmological
simulations of galaxy formation using adaptive-mesh refinement simulations of a
$10^{12}$ solar mass halo. In agreement with previous work, a run with only our
standard thermal energy feedback model results in a massive spheroid and
unrealistically peaked rotation curves. However, the addition of a simple
two-fluid model for cosmic rays drastically changes the morphology of the
forming disk. We include an isotropic diffusive term and a source term tied to
star formation due to (unresolved) supernova-driven shocks. Over a wide range
of diffusion coefficients, the CRs generate thin, extended disks with a
significantly more realistic (although still not flat) rotation curve. We find
that the diffusion of CRs is key to this process, as they escape dense star
forming clumps and drive outflows within the more diffuse ISM.
