A polystyrene-block-polyisoprene (PS-b-PI) sample with 130 styrene and 370 isoprene units was synthesized and characterized. The diblock formed mostly cylindrical micelles in N,N-dimethylacetamide with a PI core and a PS corona. The PI core of the micelles was cross-linked by S2Cl2 to yield nanofibers. The nanofibers were shortened by ultrasonication to yield fractions with weight-average length (L-w) between similar to900 and similar to3400 nm. Transmission electron microscopy and light scattering were used to characterize the fractions. The zero-shear intrinsic viscosity data [eta] of the fractions were obtained in tetrahydrofuran (THF), THF/N,N-dimethylformamide (DMF), and THF/cyclohexane (CHX), where THF is a good solvent for both the corona and the core, DMF solubilizes only the corona, and CHX is a theta solvent for the corona chains at 34.5 degreesC. The [eta] data of the fractions were treated by the Bohdanecky method derived from the Yamakawa-Fujii-Yoshizaki (YFY) theory for wormlike polymer chains and yielded the persistence length l(P) and the hydrodynamic diameters d(h) for the nanofibers. The reasonable d(h) values and the reasonable trend of d(h) variation with solvent quality change establish unambiguously the validity of YFY theory.