Self-diffusion measurements have been performed on an ordered asymmetric poly(ethylene-alt-propylene-b-dimethylsiloxane) PEP - PDMS block copolymer (M n = 30 000, f PEP = 0.79) using forced Rayleigh scattering (FRS). Samples were prepared with three different degrees of macroscopic alignment of the hexagonally packed PDMS cylinders. The diffusion coefficients parallel (D par) and perpendicular (D perp) to the cylinder axes were resolved. For a high degree of macroscopic alignment both D par and D perp were found to be significantly retarded compared to D 0, the diffusion coefficient in the absence of any interactions, D perp follows a hindered diffusion mechanism, while D par is consistent with a block retraction mechanism; both of these mechanisms have been described previously in the context of block copolymer spheres and lamellae, respectively. The extracted D perp was dependent on the degree of alignment, with the apparent D perp increasing as the degree of alignment deteriorated, whereas D par was essentially independent of the degree of alignment. This behavior of D perp is interpreted in terms of defects in the samples (i.e., both misoriented cylinders and dislocations).