  We examine the effects of the magnetic field created by the Tayler--Spruit
dynamo in differentially rotating stars. Magnetic fields of the order of a few
$10^4$ G are present through most of the stellar envelope, with the exception
of the outer layers. The diffusion coefficient for the transport of angular
momentum is very large and it imposes nearly solid body rotation during the MS
phase. In turn, solid body rotation drives meridional circulation currents
which are much faster than usual and leads to much larger diffusion
coefficients than the magnetic diffusivity for the chemical species. The
consequence is that the interplay of the thermal and magnetic instabilities
favours the chemical transport of elements, while there would be no transport
in models with magnetic field only. We also discuss the effects on the stellar
interior, lifetimes and HR diagram.
