We calculate the baryon asymmetry generated at the electroweak phase transition in the minimal supersymmetric standard model, treating the particles in a WKB approximation in the bubble wall background. A set of diffusion equations for the particle species relevant to baryon generation, including source terms arising from the CP violation associated with the complex phase delta of the mu parameter, are derived from Boltzmann equations, and solved. The conclusion is that delta must be as large as 0.1 to generate a baryon asymmetry consistent with nucleosynthesis. We criticize several recent computations of the effect, arguing that they are overestimates.