This paper deals with the optimal design of two interconnected continuous stirred bioreactors in which a single enzymatic reaction occurs. The term "optimal" should be understood here as the minimum of the total volume of the reactors required to perform a given conversion rate, given a quantity of matter to be converted per time unit. In determining the optimal volume, it is considered that the input flow may be distributed among the tanks and also that a recirculation loop may be used. The optimal design problem is solved for a wide class of kinetics functions including, in particular, the well-known Michaelis-Menten kinetics function. The analysis of the optimal configurations is investigated, and it is shown that the concept of "Steady State Equivalent Biological System". (SSEBS) first introduced by Harmand et al. [AlChE J., in press] for microbial reactions only applies to enzymatic systems which have non-monotonic kinetics. In addition, a stability as well as a sensitivity analysis of the optimal configurations are performed. (C) 2004 Elsevier Ltd. All rights reserved.