Unusual radio properties of the BL Lac object 0820+225

We present the results of simultaneous VLBA polarization observations of th e BL Lacertae object 0820+225 at 5, 8 and 15 GHz, together with earlier ima ges at 5 GHz. This source has an unusually rich total intensity and polariz ation structure compared with other objects with comparable redshifts. The magnetic field in the inner part of the complex and highly twisted VLBI jet is transverse, as is typical of BL Lacertae objects, but becomes roughly l ongitudinal further from the core, possibly as a result of shear. Although the integrated rotation measure of 0820+225 is modest, the rotation-measure distribution on parsec scales is non-uniform, and clearly shows regions wh ere the rotation measure is substantially higher than the integrated value.