Addition of a concentrated solution of PI1000-PFS50 dissolved in THF to a solution of PI1000-PFS50 seed micelles in decane led to the formation of uniform elongated fiberlike micelles with a narrow length distribution. When additional THF (>10 vol.-%) was added to the micelles, the micelle length decreased and the contour-length distribution broadened. This effect was shown to be inconsistent with a transition to an equilibrium, in which individual polymer molecules dissociated from and added to existing micelles. Rather, it appears that the polar solvent induced fragmentation of the fiberlike micelles.