During the first year of a 2-year field experiment (1989–1990), 3-year-old Scots pine (PinussylvestrisL.) seedlings were watered with phosphorus-deficient nutrient solutions containing no phosphorus (0%) or 30% of the optimal phosphorus supply. A complete nutrient solution was used as the control. In the second growing period, the seedlings were divided into two groups, a deficiency and a recovery treatment, to follow the further development of, and recovery from, symptoms. Phosphorus contents in both current- and previous-year needles in both deficiency groups decreased clearly during the first growing period. Limited growth was observed at both deficiency levels after 10 weeks of treatment. The first ultrastructural symptom, swelling of cristae and subsequent dilatation of whole mitochondria, was observed after 16 weeks of treatment in the current-year needles. In addition to this symptom, the previous-year needles had an increase in the number and translucency of plastoglobuli, a decrease of granum thylakoids, and an increase in the density of the stroma in chloroplasts. After the second growing period, the same changes were observable in the phosphorus-deficient seedlings. In the young needles grown during the recovery fertilization period, slight swelling of mitochondria was detected and recovery of the previous-year needles was not complete. The results of this experiment suggest that the swelling of mitochondria is a characteristic symptom of phosphorus deficiency at the ultrastructural level in different needle generations. All the symptoms, when occurring together, may be used in identifying a moderate phosphorus-deficiency stress in seedlings.