In the present study, a method is proposed for retrieving UV actinic flux density spectra at ground level from global and direct UV spectral irradiance measurements. The most important parameters to establish the relation between downwelling actinic flux density and global UV irradiance are the ratio of direct to global irradiance and the description of the angular distribution of the diffuse radiation field. Under clear skies, retrieved spectral actinic flux densities agreed with model calculations to within ¿5% for a variety of aerosol conditions. The assumption that the diffuse radiation is isotropic leads to a wavelength-dependent overestimation of actinic flux densities from 10 to 15%, depending on solar zenith angle, especially in atmospheres with high aerosol content. Various sources of uncertainties, which are involved in the proposed methodology, are discussed and quantified. ¿ 2000 American Geophysical Union