  Diffusion of symmetric diblock copolymer chains in macroscopically oriented
lamellar block copolymers are studied in a molecular dynamics simulation.
Results for diffusion constant both parallel $D_\parallel$ and perpendicular
$D_\perp$ to the lamellar planes are compared to results in the disordered one
phase region. For diblocks of length N=40 (1.1 N_e) and $100 (3N_e)$, where
$N_e$ is the entanglement length of a homopolymer melt at the same density,
$D_\parallel$ is nearly independent of $\chi$, while $D_\perp$ is strongly
suppressed as $\chi$ is increased. These results are in agreement with
theoretical predictions based on the Rouse model. The isotropic diffusion
constant of quenched disordered systems is approximated fairly well by
$(2D_\parallel+D_\perp)/3$ of the corresponding lamellar system.
