Natural levels of ultraviolet (UV) radiation can harm organisms in shallow aquatic ecosystems in which concentrations of photoprotective dissolved organic carbon are low(1-3). These compounds can be removed as a result of acidic precipitation and climate changes, an effect which may have recently been manifested in up to 200,000 boreal lakes(4,5). Unfortunately, meteorological and biological monitoring studies are usually too brief to record the magnitudes of past changes in UV radiation fluxes and their effects. Here we demonstrate that certain fossil pigments in lake sediments can be used to document historical changes in the UV radiation environment of lakes, These pigments are produced by benthic algae when exposed to UV radiation and show sedimentary concentrations that are correlated to the depth of penetration of UV radiation within lakes. Analysis of fossil profiles from the sediments of two mountain lakes suggests that past UV radiation penetration has sometimes been-at least in these mid-latitude lakes-greater than during the period of anthropogenic stratospheric ozone depletion.