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To understand how light attenuation in limestone rock constrains the distribution and abundance of endolithic photoautotrophic organisms, we examined light-level profiles in 25 rock samples containing endolithic algae and cyanobacteria. We collected samples from three representative cliff sites along the Niagara Escarpment, Canada, using methods that allowed us to make comparisons among as well as within sites. Photosynthetically active radiation profiles were generated by manufacturing cavities in the bottom of thick slabs of surface rock and enclosing radiation sensors in the cavities. The thickness of the flat layer of limestone between the sensor and a light source was then abraded in steps of ca. 0.5 mm. An exponential curve was fitted to the measurements for each sample to allow statistical comparisons of the light levels at different depths. We examined the correlations between physical characteristics of the rock and light attenuation and between light attenuation and the maximum depth, relative biomass, and taxonomic richness of endolithic algae and cyanobacteria. The depth at which 0.01% of available light remained varied from 2.1 to 4.5 mm at different sites, but there was large within-site variability on intermediate and small spatial scales. In contrast, the maximum depth of photoautotrophic endoliths was homogeneous within sites but different among sites (1.1-3.5 mm). Although the endoliths all have access to light, there was no significant correlation between the limit of their depth penetration and threshold quantum flux densities, indicating that the mechanisms controlling distribution and abundance of endolithic photoautotrophs are as complicated as those found for aquatic algae and cyanobacteria.