The Ozone content of the early (or paleo) atmosphere would have determined the intensity of the solar ultraviolet flux reaching the earth's surface during the early stages of biological evolution. We have calculated the ozone content of model palaeoatmospheres containing much less than the present level of atmospheric oxygen (1 PAL). The calculations take account of reactions involving water vapor, the hydroxyl and hydroperoxyl radicals, the nitrogen oxides, methane and carbon dioxide in addition to the “classical” ozone chemistry. For a wide range of assumptions covering the nature of the atmospheric model it is found that the palaeoatmosphere has a biologically effective ozone screen provided the atmospheric oxygen content is not less than 10−3 PAL. This result suggests that solar ultraviolet radiation could not have been the energy source for the prebiotic synthesis of organic molecules unless the synthesis took place outside the earth's environment.