Using Wolff's single-cluster Monte-Carlo update algorithm, the three-dimensional $O(6)$-Heisenberg model on a simple cubic lattice is simulated. With the help of finite size scaling we compute the critical exponents $\nu$, $\beta$, $\gamma$ and $\eta$. Our results agree with the field-theory predictions but not so well with the prediction of the series expansions.[ pdf] [ postscript.gz]