Much of the region of the Earth most affected by stratospheric ozone depletion is covered by a seasonal or perennial sea ice cover, which is the habitat of a productive and extensive sea ice microbial community. To assess the impact of enhanced incident ultraviolet irradiance on this community, a knowledge of the amount of light transmitted through a sea ice cover is necessary. A two-stream radiative transfer model is used to estimate the penetration of ultraviolet radiation through Antarctic sea ice. Sea ice optical properties were used as proxies to infer scattering and absorption coefficients at ultraviolet wavelengths. Case studies are reported for sea ice in McMurdo Sound and in the Weddell Sea. Values of spectral transmittance are computed as well as integrated transmitted UV-B, UV-A, biologically effective irradiance (BEI), and photosynthetically active radiation (PAR). UV-B light levels under meter-thick ice are a few percent of incident values. The presence of a snow cover results in a large decrease in transmitted ultraviolet. Snow and ice ameliorate the biological impact of enhanced levels of incident ultraviolet radiation by reducing the BEI relative to the PAR. ¿ American Geophysical Union 1993