The absorption of the solar Lyman-alpha line by the terrestrial atmosphere is calculated, taking in account the wavelength variations of the emission line and of the O2 cross-section, as well as the temperature dependence of the cross-section. A new parameterization is developed to reproduce in atmospheric models the results of this high-resolution calculation, up to an attenuation of 1010 for the incident solar radiation. The error made in most of existing models when computing the Lyman-alpha contribution to photo dissociation rates in the middle atmosphere, using a constant O2 cross-section of 10-20 cm2, is shown to be important and this can affect the loss rate of mesospheric constituents such as H2O or CH4. Copyright 1997 by the American Geophysical Union.