Wormlike micelles can form from polystyrene-b-polyisoprene copolymers in heptane-a selective solvent for polyisoprene. We have measured the aggregation number per unit length of polystyrene-b-polyisoprene wormlike micelles by light scattering and AFM. The values obtained from both techniques are in good agreement. The aggregation number per unit length decreases as the temperature increases due to the improvement of the solvent quality.