  We present an assessment of the standard model expectations for the branching
ratio of the isotopic spin and G-parity violating decay tau --> eta' pi nu_tau.
The estimate is based on a vector and scalar meson dominance parametrization of
the relevant form factors, that explicitly accounts for pi^0-eta-eta' mixing.
The numerical results obtained in this framework indicate a branching ratio one
order of magnitude (or more) below the current experimental limit, and suggest
the possibility of evidencing some novel interaction in high statistics studies
of this decay.
