  We study possible so far unaccounted isospin breaking effects in the relation
between the pion form factor as determined in e^+e^- experiments and the
corresponding quantity obtained after accounting for known isospin breaking
effects by an isospin rotation from the tau--decay spectra. In fact the
observed 10% discrepancy in the respective pion form factors may be explained
by the isospin breaking which is due to the difference between masses and
widths of the charged and neutral rho mesons. Since the hadronic contribution
to the muon anomalous magnetic moment can be calculated directly in terms of
the e^+e^- - data the corresponding evaluation seems to be more reliable. Our
estimate is a_mu(had)=694.8(8.6) x 10^{-10}. The tau - data are usefull at the
presently aimed level of accuracy only after appropriate input from theory.
