The longwave radiation budget at the top of the atmosphere (TOA) and at the surface of the Southern Hemisphere was estimated for 10¿ latitudinal zones, using a radiative transfer model and long-term mean monthly climatological data from the International Satellite Cloud Climatology Project (ISCCP) from 1983 to 1990. The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) scanner S4 satellite data (1984--1989) while at the surface they have been compared with other model results and available surface measurements. Large discrepancies were found between model and ERBE TOA fluxes over Antarctica that were resolved by increasing the winter polar cloudiness significantly. Generally, our model TOA fluxes agree within the standard deviation of the ERBE data. Important differences from previous works are also found in the polar regions, in terms of magnitudes, and seasonal/latitudinal trends in surface fluxes. The mean annual and hemispherical outgoing longwave radiation at TOA, upward longwave radiation, downwelling, and net upward longwave radiation at the surface, are computed to be 236, 387, 341, and 46 W m-2, respectively. ¿ 2001 American Geophysical Union