A fully automatic computer-controlled video analysis system has been used to study the movement of the green unicellular flagellate, Euglena gracilis in a horizontal or vertical cuvette. In darkness, in the absence of gaseous gradients, most cells swim straight upwards. While in a horizontal cuvette the transition between positive and negative phototaxis is found at about 1.5 W m-2, an excess of 30 W m-2 is required to reverse the upward swimming (due to the combined stimulus of negative gravitaxis and positive phototaxis) in a vertical cuvette. By studying the swimming direction in horizontal and vertical cuvettes in polarized light irradiated from above or from the side, respectively, the dichroic orientation of the photoreceptor molecules can be determined in three dimensions with respect to the axes of the cell; In a horizontal cuvette, in a linearly polarized beam from above, the cells orient predominantly at an angle of about 30 degrees clockwise off the electric dipole transition moment as seen from above. The behavior in a vertical cuvette with polarized light entering from above indicates that the photoreceptor pigments are dichroically oriented 60 degrees counterclockwise from the flagellar plane (seen from the front end of the cell). Experiments with horizontal polarized light indicate that the photoreceptor transition moment deviates 25 degrees clockwise off the long axis of the cell.