Cells of the halotolerant microalga Dunaliella parva immobilized in calcium-alginate gel showed morphological characteristics and photosynthetic activity similar to those of free cells. The photosynthetic activity in immobilized cells fell only about 20% during 2 weeks incubation at 5 degrees C in the light. The asymmetric reduction of hydroxyacetone to (R)-propanediol was first examined in batch reactions with both free and immobilized cells. Since the activity of immobilized cells was as high as that of free cells, the optimum conditions for the column reactor were investigated using immobilized cells. The productivity of propanediol in immobilized cells increased in proportion to increases in the light intensity, suggesting that NADPH regenerated through photosynthesis was used for the asymmetric reduction. Electron microscopic observation of thin sections of immobilized cells after the reaction suggested that NADPH used for asymmetric reduction in the dark was produced through the metabolism of starch granules. The use of a CaCO3-free medium in the column reactor markedly prolonged the period of reductive activity and photosynthesis.