ABSTRACT Potassium-on-copper vacuum photo-electric cells, in conjunction with a thermionic potentiometer and Unipivot galvanometer, have been found to be satisfactory for measuring sub-aqueous light intensities. Some characteristic results indicate that the light intensity at the limit of sub-aqueous vegetation may be much higher than those recorded in corresponding terrestrial habitats. It is suggested that the quality of the light is of great biological importance.