Influences of phenol, α-naphtol and β-naphtol, which are toxic chemicals, on citric acid biosynthesis and biomass in the artificial culture setting of Aspergillus niger using batch fermenter are examined in the most favorable fermentation conditions, and a model is proposed. Addition of certain concentrations of phenol, α-naphtol and β-naphtol to the culture increases the citric acid production. According to this model, maximum citric acid concentration is 48.3 g L^-1 in a culture that does not contain any toxic chemicals, whereas the maximum concentrations obtained in cultures containing 25 mg L^-1 phenol, 25 mg L^-1 alpha-naphtol and 15 mg L^-1 beta-naphtol are 62.5 g L^-1, 78.1 g L^-1 and 86.0 g L^-1, respectively. Moreover, addition of toxic chemicals to the culture reduces fermentation time by 24 hrs.