[en] The polarization parameter in π^-p→K⁺Σ^- backward scattering has been measured at 3.5 GeV/c incident momentum, in the u-range -0.6< u<0.1 GeV². The apparatus consisted of a polarized target in a spectrometer magnet, counter hodoscopes for time-of-flight measurements, a larger water Cerenkov counter for kaon tagging, and wire spark chambers. After identification of the kaons, the reaction was selected by a missing-mass technique. An isospin analysis has been carried out to separate the contributions of the various isospin exchange amplitudes in the u-channel. (Auth.)