Chloroplast ultrastructure was studied during the season of main growth on leaves of Deschampsia flexuosa (L.) Trin. collected from the wild in South Finland. Corresponding leaf segments were used for concurrent measurements of the O2 evolution rate, chlorophyll (chl) content and chl a/b ratio. No significant diel variation was observed in the content or arrangement of grana thylakoids but fluctuation was evident in the volume of starch grains. The seasonal changes in thylakoid organization were more marked. At the beginning of June higher values than in typical sun species were recorded for the ratio of the length of appressed to non-appressed thylakoid membranes (1.8), the average number of partitions per granum (9.5) and the grana area (31 % of the chloroplast area). All these parameters increased as the season progressed, especially after the middle of July. The chl content and chl a/b ratio decreased until the middle of August, but later in August, after rain, new leaves had a chl content and chl a/b ratio as high as at the beginning of June. In contrast, chloroplast ultrastructure remained typical for this late growth period. The highest photosynthetic capacity was measured during the two first weeks in June. At the beginning of July this capacity had decreased by 61%, and stayed low until the end of the experimental period. The changes in chloroplast ultrastructure are discussed in relation to the photosynthetic activity and the composition of the chlorophyll-protein complexes in appressed and non-appressed thylakoid membranes.