The evolution of polystyrene-polyisoprene block copolymer self-assembly is studied in coaxially electrospun nanofibers via annealing. The leftmost image shows the cross-sectional and longitudinal sections of as-spun fibers. Moving to the right, a transition of morphology can be seen from stacked disk structures for lower annealing temperatures to alternating concentric cylinders of polyisoprene and polystyrene for higher annealing temperatures and times.