A microphase-separated core-shell cylinder morphology has been observed, via transmission electron microscopy and small-angle X-ray scattering, in diblock copolymers of polystyrene (PS) and poly(1,3-cyclohexadiene) (PCHD). The structures consist of PS cylindrical cores surrounded by PCI-ID cylindrical annuli which are then hexagonally packed in a matrix of PS. The materials were produced by anionic polymerization of styrene followed by 1,3-cyclohexadiene. Characterization by size exclusion chromatography revealed a main peak due to diblock with a vet-Si narrow molecular weight distribution. However, a significant amount of PS homopolymer (about 30%) was present in the as-synthesized materials. The as-synthesized materials with homopolymer present produced core-shell morphologies, and these structures became much more regular when the homopolymer uas removed by fractionation. After fractionation, the pure core-shell forming diblocks had PCHD volume fractions of around 0.37 and polydispersities well under 1.1.