The vertical distribution of density or temperature is studied in turbulent boundary layers stratified by either salt or temperature and driven in an annulus by a rotating screen. With rapidly growing boundary layers the buoyancy flux due to entrainment becomes sufficiently large that the boundary layer is no longer well mixed. Stratification in the boundary layer is largest near the entrainment interface, corresponding to extension of the density transition into the turbulent boundary layer. The stratification in the rest of the boundary layer is smaller and nearly independent of height. The mean boundary layer stratification and structure and integral properties of the density distribution are found bo be strongly dependent on the ratio of the entrainment rate of the turbulent velocity scale.