  We confirm the reality of a reversal of the sign of the Faraday Rotation
Measure in the Galactic plane in Cygnus (Lazio et al, 1990), possibly
associated with the Cygnus OB1 association. The rotation measure changes by
several hundred rad/m$^2$ over an angular scale of $2-5^{\circ}$. We show that
a simple model of an expanding plasma shell with an enhanced density and
magnetic field, consistent with observations of H$\alpha$ emission in this part
of sky, and physically associated with a superbubble of the Cygnus OB1
association, can account for the magnitude and angular scale of this feature.
