The biomass of the endolithic algae Ostreobium quekettii Phyllosiphoniaceae living within skeletons of the scleractinans Mycedium elephantotus and Leptoseris fragilis averages 300 μg protein. cm-2. This represents approximately 7% of the protein of the zooxanthellae-containing tissue of M. elephantotus and approximately 38% of that of L. fragilis. Oxygen production Pmaxnet of 0. querkettii in bare skeletons of M. elephantotus averaged 0.7 μg O2.cm-2· h-1 measured in large skeletal fragments. This amount is approximately 6% of the productivity of the zooxanthellae Symbiodinium microadriaticum living in the same scleractinian species at the same depth Pmaxnet 11 μg O2· Cm-2· h-1. Light compensation of O. quekettii - within skeletons - was reached at approximately 10 and saturation at 35 40 μE·m-2· s-1. Algae within the M. elephantotus skeletons receive a maximum of 4–6% of the ambient irradiance, which is approximately 0.9 μE · m-2· s-1 approximately 0.04% surface irradiance at a depth of 88 m. In L. fragilis at a depth of 145 m, the photon flux decreases to 0.3 μE·m-2· s-1, which is less than 0.004% of surface intensity. With increasing depth, the ratio of Chl b to Chl a increased in endolithic algae colonizing L. fragilis, indicating improvement of light harvesting under low light conditions. In free-living O. quekettii cultured at irradiance levels from 0.5–60 μE·m-2· s-1, the concentrations of chlorophylls increased and that of siphonein and β-carotene decreased with decreasing photon flux.