  The asymptotic frequency $\omega$, dependence of the dynamic viscosity of
neutral hard sphere colloidal suspensions is shown to be of the form $\eta_0
A(\phi) (\omega \tau_P)^{-1/2}$, where $A(\phi)$ has been determined as a
function of the volume fraction $\phi$, for all concentrations in the fluid
range, $\eta_0$ is the solvent viscosity and $\tau_P$ the P\'{e}clet time. For
a soft potential it is shown that, to leading order steepness, the asymptotic
behavior is the same as that for the hard sphere potential and a condition for
the cross-over behavior to $1/\omega \tau_P$ is given. Our result for the hard
sphere potential generalizes a result of Cichocki and Felderhof obtained at low
concentrations and agrees well with the experiments of van der Werff et al, if
the usual Stokes-Einstein diffusion coefficient $D_0$ in the Smoluchowski
operator is consistently replaced by the short-time self diffusion coefficient
$D_s(\phi)$ for non-dilute colloidal suspensions.
