A kinetic model was proposed to represent the profile of pigment production associated with the growth of red beet hairy roots. This model was constructed on the basis of a concept of cellular age distribution arising from a linear growth mode of the roots. The kinetics of root proliferation were formulated using a Monod-type rate equation of root elongation with intracellular phosphorus as a limiting substrate. A positional dependence of pigment content was observed along with the hairy roots; namely, content increased with increasing distance from the root tips and gradually approached a saturated value correlated with intracellular phosphorus content. The kinetics of pigment formation of hairy roots were presented by considering the variation in pigment content along the roots and the intracellular phosphorus effect. It was demonstrated that the model made it possible to describe the kinetic behaviors of growth and pigmentation during hairy root culture in a fermenter.