Single amoebae of Dictyostelium discoideum were locally stimulated with microbeams of white and monochromatic light. Low illuminance stimulation favored formation of pseudopodia at the irradiated parts of the cells, high illuminance stimulation locally suppressed the extension of pseudopodia. When the high illuminance light spot was placed on any portion of the cell other than the moving front, no response could be observed. The results are compatible with the assumption that, during their phototactic response, single amoebae detect the direction of light by a shadowing effect caused by pigments like cytochromes, and/or by light scattering of particles in the cytoplasm.