We present 1420 MHz polarization images of a 2:5 ; 2:5 region around the planetary nebula (PN ) Sh 2-216. The images are taken from the Canadian Galactic Plane Survey (CGPS). An arc of low polarized intensity (size 0:2 ; 0:7 ) appears prominently in the northeast portion of the visible disk of Sh 2-216, coincident with the optically identified interaction region between the PN and the interstellar medium (ISM). The arc contains structural variations down to the $1 0 resolution limit in both polarized intensity and polarization angle. Several polarization-angle ''knots'' appear along the arc. By comparison of the polarization angles at the centers of the knots and the mean polarization angle outside Sh 2-216, we estimate the rotation measure (RM) through the knots to be À43 AE 10 rad m À2 . Using this estimate for the RM and an estimate of the electron density in the shell of Sh 2-216, we derive a line-of-sight magnetic field in the interaction region of 5:0 AE 2:0 G. We believe it more likely the observed magnetic field is interstellar than stellar, although we cannot completely dismiss the latter possibility. We interpret our observations via a simple model which describes the ISM magnetic field around Sh 2-216, and comment on the potential use of old PNe as probes of the magnetized ISM. Subject headingg s: ISM: structure -planetary nebulae: individual (Sh 2-216) -polarizationradio continuum: ISM Online material: color figure