The optimum design of a series of continuously operated stirred-tank reactors containing immobilised growing cells is described. Optimal design is defined as the minimal total holding time over the reactor series to achieve a certain degree of conversion. The analysis is made under the assumptions that there is a constant and equal concentration of immobilised biomass in all bioreactors along the series, no diffusion limitation takes place, all growth of the immobilised biomass will lead to an increase in suspended biomass, and that maintenance of the immobilised cells can be neglected. It is shown that the use of more than three bioreactors in series,is likely to be de trop.