  The present article determines the indices of the principal congruence
subgroups of the Bianchi groups $B_d$, $SL(2,\mathcal O)$ and elementary matrix
group $E$ and extends Wohlfahrt's Theorem to $B_d$, $SL(2,\mathcal O)$ and $E$,
where $\mathcal O$ is the ring of integers of some number fields.
