The fine granularity of the CHARM-II detector has been exploited to search, in the CERN-SPS wide band neutrino beam, for quasi-elastic nu(tau) interactions followed by the decay tau --> pinu(tau). Since the sampling thickness of the target calorimeter corresponds to approximately 1/9 of an interaction length, these events appear in the detector as a single track followed by a hadronic shower. The study of the ''single pion'' events is used to set limits on the nu(mu) --> nu(tau) oscillation parameters. The maximum sensitivity to the mixing angle theta is reached for DELTAm2 = 50 eV2 allowing to exclude values of sin2 20 greater than 6.4 x 10(-3) at 90% CL.