A fuzzy control system featuring the identification of culture phases by fuzzy logic was applied to the large-scale recombinant vitamin B-2 (VB2) production at the Fukuroi factory of Nippon Roche, Fukuroi. Prior to actual operation, the knowledge base for fuzzy control was tuned and verified by a simulation system. Only four rules, formulated as IF similar to THEN instructions and membership functions for state variables, are employed to control the feed rate and pH of the fed-batch culture. By applying fuzzy control, total VB2 production and VB2 yield was increased by 6-16% and 4-11% compared with conventional control, respectively. Operating stability was also improved. The large-scale fermenter used for VB2 production has been successfully operated under the control of the fuzzy system for more than 2 years.