Continuous production of phospholipase D (PLD) in an air-lift reactor system was investigated using growing cells of Streptomyces lydicus D-121 immobilized within cross-linked chitosan beads as porous supports. In a continuous culture with the immobilized growing cells, both the activity and productivity were highest at a dilution rate of 0.330 h(-1) which was higher than the maximum specific growth rate of the free cells (0.247 h(-1)). The highest PLD activity was obtained at pH 6.0-6.5. The average activity was 12.7 U/ml during 120-384 h in a continuous culture with immobilized cells, while the productivity of the continuous culture (4.19 x 10(3) U/l . h) was about 3.3-3.6-fold higher than that of the batch culture (1.16 x 10(3) U/l . h) and the repeated-batch culture in the reactor (1.27 x 10(3) U/l . h), and about 3-fold higher than that in the case of continuous culture with free cells (1.36 x 10(3) U/l . h).