Regeneration of NADPH via photosynthetic pathway was applied to the development of a novel bioreactor in which NADPH is required for the desired reaction. In a preliminary experiment using dihydroxyacetone reductase (DHAR) from Dunaliella parva (D. parva), hydroxyacetone (HA, Acetol) was reduced to yield (R)-(-)-1,2-propanediol ((R)-PD) with an optical purity of 75%ee. The bioconversion using D. parva whole cells in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) showed that the NADPH required for this reaction can be generated by photosynthesis. The optimum reactor conditions for propanediol (PD) production were 50-100 mM HA at 25 degrees C. The production rate of PD increased proportionally with increasing irradiation intensity up to 220 mu E . s(-1). m(-2). Although changes in cell density did not affect the productivity, agitation of the reaction mixture at 200 rpm raised the productivity. Since the amount of PD produced was almost the same as the amount of HA consumed, it was confirmed that more than 95% of the HA was converted to PD without turning into any by-products or being degraded. The PD productivity in the photobioreactor system using Dunaliella cells was comparable to that in the yeast cell system.