The energy band gap narrowing (BGN) as a function of doping concentration in Be-doped Al0.33Ga0.67As is investigated using photoluminescence (PL) measurements on samples grown by molecular beam epitaxy (MBE). An expression for theoretically predicting the band gap narrowing has been deduced for p-type doping in AlxGa1-xAs (0 less than or equal to x less than or equal to 0.45). The experimental results obtained from the PL spectra are in good agreement with this expression. A simple empirical relation for the PL peak energy of Be-doped Al0.33Ga0.67As as a function of carrier concentration is also presented. This provides a convenient way of determining the carrier concentration in Be-doped Al0.33Ga0.67As samples nondestructively.