1. The fluorescence spectra of the alga Porphyridium have been recorded as energy distribution curves for eleven different incident wave lengths of monochromatic incident light between wave lengths 405 and 546 mµ.
2. In these spectra chlorophyll fluorescence predominates when the incident light is in the blue part of the spectrum which is strongly absorbed by chlorophyll.
3. For blue-green and green light the spectrum excited in Porphyridium contains in addition to chlorophyll fluorescence, the fluorescence bands characteristic of phycoerythrin and of phycocyanin.
4. From these spectra the approximate curves for the fluorescence of the individual pigments phycoerythrin, phycocyanin, and chlorophyll in the living material have been derived and the relative intensity of each of them has been obtained for each of the eleven incident wave lengths.
5. The effectiveness spectrum for the excitation of the fluorescence of these three pigments in vivo has been plotted.
6. From comparisons of the effectiveness spectrum for the excitation of each of these pigments it appears that both phycocyanin and chlorophyll receive energy from light which is absorbed by phycoerythrin.
7. It is suggested that phycocyanin may be an intermediate in the resonance transfer of energy from phycoerythrin to chlorophyll.
8. Since phycoerythrin and phycocyanin transfer energy to chlorophyll, it appears probable that chlorophyll plays a specific chemical role in photosynthesis in addition to acting as a light absorber.