In the present work, we employ a fuzzy logic controller (FLC) to contr
ol the unstable state of a nonlinear biological reaction. The state va
riable vectors consist of cell density and substrate concentration. Th
e dilution rate is used as a manipulated variable to control the react
ion dynamics. An analytic form of FLC employing Zadeh AND logic along
with Center of Mass defuzzification method is considered. Simulations
reveal that for servo response test, the FLC shows satisfactory perfor
mance for natural unsteady states for which a conventional PI controll
er is known to fail. Further simulations also show that the FLC gives
satisfactory regulatory response and is relatively insensitive to the
deviations in model parameters.