Photovoltaic cells of the type (Al/monolayers of photosynthetic pigments/Ag) have been prepared by sandwiching pigment monolayers between semitransparent Al and Ag electrodes. The use of monolayers is to simulate the in vivo orientation of pigments. The power conversion efficiency of chlorophyll cells measured at 680 nm (red absorption band) is 0. 03%. The addition of quinone or carotenoid canthaxanthine does not seem to increase the efficiency of the cells. The capacitance measurements at low frequency in the dark and under illumination indicate the presence of trapped charge carriers which are mobilized by light. The action spectra suggest that the photoactive region is at the Al/pigment junction.