[en] The polarization parameter has been measured for π^-p elastic scattering in the backward region at 3.5 GeV/c incident momentum. The experimental set-up consisted of a polarized target in a spectrometer magnet, hodoscopes and wire spark chambers. Data are presented for the range -0.95< u<=-0.19 GeV². An isospin analysis has been carried out to separate the Isub(u)=1/2 and Isub(u)=3/2 contributions. (Auth.)