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Abstract. In epidermal cells of the leaves of the aquaticangiosperm Vallisneria gigantea Graebner, the chloroplasts accumulate in the outer periclinal layer of cytoplasm (P side) under light at low fluence rates. The nature of such intracellular orientation of chloroplasts was investigated in a semiquantitative manner. Time-lapse video microscopy revealed that, while irradiation with red light (650 nm, 0.41 W" m-2) rapidly accelerated the migration of chloroplasts, not only from the anticlinal layers of cytoplasm (A sides) to the P side but also from the P side to the A sides, the increased rate of migration in both directions returned to the control rate upon subsequent irradiation with far-red light (746nm, 0.14W.m-2). These effects of red and far-red light could be observed repeatedly, both in the presence and in the absence of inhibitors of photosynthesis, suggesting the involvement of phytochrome as the photoreceptor. After saturating irradiation with red light, the increased rate of migration of chloroplasts from the P side to the A sides declined more rapidly than the increased rate of migration in the opposite direction. This imbalance in the migration of chloroplasts between the two opposing directions resulted in the accumulation of chloroplasts on the P side. The more rapid decline in the rate of migration of chloroplasts from the P side to the A sides than in the opposite direction was not observed in the presence of an inhibitor of photosynthesis. It appears, therefore, that phytochrome and photosynthetic pigment cooperatively regulate the accumulation of chloroplasts on the P side through modulation of the nature of the movement of the chloroplasts.Abbreviations: A side = cytoplasmic layer that faces the anticlinal wall; DCMU = 3-(3,4-dichlorophenyl)-l,l-dimethylurea; Pfr = farred-light-absorbing form of phytochrome; Pr = red-light-absorbing form of phytochrome; P side = cytoplasmic layer that faces the outer periclinal wall Correspondence to: S. Takagi; FAX: 81 (6) 850 5817; Tel: 81 (6) 850 5818