Red beet hairy roots are cultivated in a single column reactor to investigate the influence of superficial velocity of medium on formation of betanin (a major pigment component in red beet hairy roots). The betanin content in the cells increases with increasing superficial velocity in the range of 13 to 40 m/h, In the cultures at superficial velocities of 15 and 40 m/h for 166 h, the betanin content increases with elapsed time of cultures. The growth of the hairy roots is responsible for an increase in shear stress on the root surfaces By means of lowered void fraction of hairy root hed, and the estimated values of shear stress range from 0.22 to 188 N/m(2) during both the cultures. The activity of monophenol monooxygenase in the cells, which is a key enzyme involved in the biosynthesis of betanin, can be correlated with the value of shear stress in such a manner as to approach plateaus gradually with increasing shear stress.