  We investigate the Rouse dynamics of a flexible ring polymer with a prime
knot. Within a Monte Carlo approach, we locate the knot, follow its diffusion,
and observe the fluctuations of its length. We characterise a topological time
scale, and show that it is related to a self-reptation of the knotted region.
The associated dynamical exponent, $z_T=2.32\pm.1$, can be related to that of
the equilibrium knot length distribution and determines the behaviour of
several dynamical quantities.
