Summary In the green alga Mougeotia , different types of phytochrome mediated chloroplast movements are known. They result in face or profile position of the chloroplast depending on the orientation of the electrical vector of polarized light. These responses require only a single short light puls and they are completed within half an hour. In contrast to these «short-term movements», a «long-term movement» has been discovered and will be investigated in the present paper: If polarized red light, vibrating parallel to the long axis of the cell, is given continuously during several hours, the chloroplast turns very slowly into profile position. This response is not due — as might be suspected — to scattered light from the environment of the cell (Fig. 5 and 6) or from within the cell. Hence we are dealing with a real orienting effect of the parallel vibrating light. With increasing irradiation time, a first maximum (percentage of profile position) is reached at about 3–4 hours, followed by a minimum at about 20 hours. With still longer irradiations, a new orientation to the profile position is observed which is obvious after 3 days (Fig. 7 and 9) and which reaches 100 percent after a few weeks. The times required to obtain the first maximum and the minimum are independent of the red light intensity, provided there is any response at all; but the scope of the response curve depends on the light intensity (fig. 7). More detailed investigations have been restricted to the first increase until 200 min. Since continuous red light can be replaced by repeatedly given red pulses, red/far-red reversibility can be tested. From these experiments (Fig. 11), phytochrome has been demonstrated as the photoreceptor. Accordingly, long wavelength red («middle-red», 690 to 695 nm) is less effective than «red» light (667 nm) (Fig. 10), but even the green «safe light» (537 nm) is not completely without an effect in the long term response (Fig. 2). As to the perception of light direction, the following assumption has been made: By attenuation of the light in the chloroplast, the «rear» of the cell obtains less light than the other regions, and this darkest region will attract one edge of the chloroplast. This is confirmed by the observation that in the turning movement that edge always preceds which approaches the rear (Fig. 13). Further interpretations have to match two facts which seem to be contradictory: From the red/far-red experiments, it has been concluded that P fr (F in this paper) controls the response. But the long-lasting irradiations result in photostationary conditions in all parts of the cell, hence no gradient of P fr . To account for these different findings, two assumptions have been made: 1) The physiological gradient responsible for the movement is established by local differences in the rate of Pfr formation, and hence of phytochrome cycling P r ⇋P fr (Fig. 12). 2) This gradient of phytochrome cycling can act only if the overall P fr concentration is sufficiently high. On the basis of these assumptions, the relative spectral sensitivity of the response can be predicted, taking into consideration the absorption gradient, the rate of cycling, and the photostationary P fr concentration (Tab. 2), and the experimental results are in good agreement. Some speculations are added about the interaction of P fr with phytochrome cycling and about the mechanism of movements as controlled by these factors.