Control parameters directly connected with metabolism of microorganism that reflect its actual activity in gluconic acid biosynthesis can be chosen basing on mass balance of the process. The relatively high correlation between parameters, normally determined via sample analysis and microorganisms? respiratory activity, enabled on-line estimation of biomass growth, product formation, and substrate uptake rate. The estimated parameters were continuously measured and employed to on-line control of gluconic acid overproduction by Aspergillus niger W78B strain. The application of the noninvasive technique for parameters determinations allowed obtaining high effectiveness of product formation characterized by the yield coefficient that equals 1.0 g g-1 and product formation rate amounted to 5 g dm-3 h-1. The estimation of the parameters cannot be used instead of their quantitative determinations, but it gives a possibility of on-line monitoring and control of the process.