We present 1420 MHz polarization images of a 2.5 \arcdeg \times 2.5 \arcdeg 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 \arcdeg \times 0.7 \arcdeg ) appears prominently in the north-east 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 \sim 1′ 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 RM through the knots to be -43 \pm 10 rad m ^ { -2 } .
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 \pm 2.0 \muG .
We believe it more likely the observed magnetic field is interstellar than stellar , though we can not 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 .