Responses using light as a sensory stimulus for orientation are of elementary importance not only for the photosynthetic flagellates, but also for a number of non-photosynthetic ciliates, which may utilize this behaviour e.g. to approach their prey, or to escape their predators. Phototactic microorganisms generally have developed highly specialized structures which they use for the detection of the light direction. Ultrastructure and function of the photoreceptive apparatuses are closely linked together. In order to explain the role of cell organelles suspected of being the photoreceptive apparatuses of phototactic protozoa, different hypotheses have been proposed, suggesting that some organelles either act as light absorbing or as light reflecting elements. As a result, during rotation of a cell, light is modulated by strong fluctuations in the irradiance. If the photoreceptor molecules were located near such a light modulating organelle, a directional antenna would continuously scan the environment thus enabling the cell phototactic orientation. The chemical nature of a few photoreceptor molecules has been elucidated.