Fuzzy set theory is applied to extend the optimal control problem for complex objects such as biotechnological processes. It allows inclusion of all possible and admissible process states in the problem formulation. A new approach for model representation with convenient fuzzy sets taking into account expert experience is proposed. Bellman-Zadeh's approach is applied to transform a fuzzy optimal control problem ot a deterministic one which is solved numerically. Optimal control of a real-life fermentation process is determined and a better solution is reached due to more realistic and flexible problem formulation. (c) Elsevier