SUMMARYThe effects of aqueous SO2 on the ultrastructural organization of pine (Pinus contorta Dougl. var. latifolia Engelm.) chloroplasts and on their photosynthetic activity were determined under laboratory conditions. At aqueous concentrations of 100 and 500 ppm, SO2 caused swelling of thylakoid discs and disintegrated other intrachloroplast membranes, resulting in the formation of small vesicles (in older, matured tissues). Chloroplast structural injury was more pronounced in old tissues than in younger and more metabolically active tissues. The biochemical observations (Hill reaction activity) made on chloroplasts isolated from SO2‐treated pine needle segments are in good agreement with the cytological observations.