The modified two-stream approximation (MTSA) of the radiative transfer equation is further developed and applied to the calculation of the downward scattered solar ultraviolet radiation reaching the ground. This MTSA solution is parameterized and fitted to the results obtained from a multistream scattering code. To facilitate practical application, parameters introduced into the MTSA formular allow for variations in the concentration of ozone, wavelength, and solar zenith angle. The effect of ground albedo on the UV flux is also discussed. This formula should be useful in a variety of applications related to the photobiologically important solar UV radiation reaching the ground.