  We derive a new expression for the coefficient $D_{\mathrm{h}}$ of diffusion
by horizontal turbulence in rotating stars. This new estimate can be up to two
orders of magnitude larger than given by a previous expression. As a
consequence the differential rotation on an equipotential is found to be very
small, which reinforces Zahn's hypothesis of shellular rotation. The role of
the so--called $\mu$--currents, as well as the driving of circulation, are
reduced by the large horizontal turbulence. Stellar evolutionary models for a
20 M${\odot}$ star are calculated with the new coefficient. The new and large
$D_{\mathrm{h}}$ tends to limit the size of the convective core and at the same
time it largely favours the diffusion of helium and nitrogen to the surface of
rotating OB stars, a feature supported by recent observations.
