  Semi-convection is a slow mixing process in chemically-inhomogeneous
radiative interiors of stars. In massive OB stars, it is important during the
main sequence. However, the efficiency of this mixing mechanism is not properly
gauged yet. Here, we argue that asteroseismology of $\beta$ Cep pulsators is
capable of distinguishing between models of varying semi-convection
efficiencies. We address this in the light of upcoming high-precision space
photometry to be obtained with the Kepler two-wheel mission for massive stars
along the ecliptic.

Solar and Stellar Astrophysics