  We describe isomorphism patterns in the $p$-primary part of the Farrell
cohomology ring ${\hat{H}}^*(Sp(p-1,Z[1/n]),Z)$ for any odd prime $p$ and
suitable integers $0\neq n\in Z$, where $Sp(p-1,Z[1/n])$ denotes the group of
symplectic $(p-1)\times (p-1)$--matrices. Moreover, we determine the precise
$p$-period of this ring.
