A simple and rapid calculation method, which is based on the Beer-Lambert law and Rose and Lloyd's hypothesis, was proposed for the determination of light attenuation profiles in suspensions of photosynthetic microorganisms. In this method, extinction coefficients measured spectrophotometrically at 2 single concentration of suspended cells over the whole range of wavelengths emitted from different light sources were used for reflecting the physical and biological characteristics of cells, which may change during a course of cultivation. The validity of this approach was confirmed by use of suspensions of two kinds of photosynthetic cells, the purple bacterium Rhodobacter capsulatus and the green alga Chlamydomonas reinhardtii, illuminated by three light sources with different irradiation spectra. This approach can estimate well the profiles of the light attenuation in these systems, irrespective of the spectra of the light sources, until the transmitted light intensity falls to one tenth of the incident intensity.