There are remarkably few reports on D-arabitol production from lactose. Previous studies in our laboratory have shown that the osmophilic yeast Kluyveromyces lactis NBRC 1903 convert lactose to extracellular D-arabitol. The present study was undertaken to determine the participation of osmotic stress caused by lactose on D-arabitol production by K. lactis NBRC 1903 and to provide the information on the kinetics of D-arabitol production from lactose by K. lactis NBRC 1903. It was confirmed that D-arabitol production was triggered when an initial lactose concentration was above 278 mmol L(-1). D-Arabitol yield increased with an increase in initial lactose concentration. The highest D-arabitol concentration of 79.5 mmol L(-1) was achieved in the cultivation of K. lactis NBRC 1903 in a medium containing 555 mmol L(-1) lactose and 40 g L(-1) yeast extract. Lactose was found to play two important roles in D-arabitol production by K. lactis NBRC 1903 grown on lactose. First, lactose was assimilated as the substrate both for cell growth and D-arabitol production. Second, a high lactose concentration induced cellular response to high osmotic stress and up-regulated the flow from D-glucose-6-phosphate to D-arabitol. The arrest of cell growth triggered D-arabitol production.