The microphase separation in a linear ternary block copolymer consisting of three immiscible components, namely, polystyrene, polybutadiene, and poly(methyl methacrylate), gives rise to the formation of a chiral supramolecular assembly in which the chirality is not defined on a molecular (or macromolecular) level but only by the assembly of the linear triblock copolymer molecules. The polybutadiene midblock (7 and 12 wt %) forms helical strands surrounding the polystyrene cylinders (25 and 26 wt %) which are embedded in a poly(methyl methacrylate) matrix. This chiral mesoscopic assembly can be recognized by transmission electron microscopy. The formation of this structure is controlled by the relative incompatibilities of the three polymeric components in the block copolymer.