We report a study of the decay τ−→τ−τ+τ−vτ using the ARGUS detector at the DORIS II storage ring. Tau pairs were produced bye+e− annihilation at centre-of-mass energies near 10 GeV. We have observed about 1,700 events, corresponding to a branching ratio for τ−→τ−τ+τ−vτ of (5.6±0.7)%. The dominant mode for this decay is observed to beρ0π− in aJp=1+ spin state. Theρ0π− mass spectrum shows a broad enhancement near 1.10 GeV/c2, which has the properties expected for theA1.