The polarization of - hyperons produced by 400-GeV protons in the reaction p+Be-+X has been measured as a function of momentum at two production angles. The average polarization for the full sample (192 110 events) was -0.108±0.007. Comparisons are made with polarization measurements for other hyperons produced under similar conditions. From the same data, was measured to be -0.303±0.004±0.004, where ± is the asymmetry parameter in the decay p-, ± is the asymmetry parameter in the decay --, and the uncertainties are statistical and systematic, respectively. This yields ±=-0.472±0.006±0.011, where the systematic uncertainty is dominated by the uncertainty in ±. An updated test of the I=1/2 rule in decay is presented.