The application of a Fresnel-based fiber sensor on real-time monitoring of the fermentation process in bioethanol production is reported. The fiber was placed inside the bioreactor, and experiments were conducted by using glucose solution and sugarcane syrup as substrates for fermentation. When the sugar is completely consumed, there is no production of ethanol, causing the sample concentration to become constant, as well as the reflected light intensity. Therefore, the sensor can be used to predict the ideal moment to terminate the process. The results were confirmed by additional laboratory analysis, making this an alternative technology for optimization of bioethanol production. © 2012 SPIE.8421 Goldemberg, J., Guardabassi, P., The potential for first generation ethanol production from sugarcane (2010) Biofuels, Bioprod. Bioref., 4 (1), pp. 17-24Dias, M.O.S., Junqueira, T.L., Cavalett, O., Cunha, M.P., Jesus, C.D.F., Rossell, C.E.V., Filho, R.M., Bonomi, A., Integrated versus stand-alone second generation ethanol production from sugarcane bagasse and trash (2012) Bioresource Technol., 103 (1), pp. 152-161Leite, R.C.C., Leal, M.R.L.V., Cortez, L.A.B., Griffin, W.M., Scandiffio, M.I.G., Can Brazil replace 5% of the 2025 gasoline world demand with ethanol? (2009) Energy, 34 (5), pp. 655-661Rivera, E.C., Costa, A.C., Atala, D.I.P., Maugeri, F., Maciel, M.R.W., Filho, R.M., Evaluation of optimization techniques for parameter estimation: Application to ethanol fermentation considering the effect of temperature (2006) Process Biochem., 41 (7), pp. 1682-1687McNeil, B., Harvey, L.M., (2008) Practical Fermentation Technology, p. 388. , John Wiley & Sons, ChichesterVeale, E.L., Irudayaraj, J., Demirci, A., An on-line approach to monitor ethanol fermentation using FTIR spectroscopy (2007) Biotechnol. Prog., 23 (2), pp. 494-500Cavinato, A.G., Mayes, D.M., Ge, Z., Callis, J.B., Noninvasive method for monitoring ethanol in fermentation processes using fiber-optic near-infrared spectroscopy (1990) Anal. Chem., 62 (18), pp. 1977-1982Culshaw, B., Optical fiber sensor technologies: Opportunities and-perhaps-pitfalls (2004) J. Lightwave Technol., 22 (1), pp. 39-50Fujiwara, E., Ono, E., Manfrim, T.P., Santos, J.S., Suzuki, C.K., Measurement of sucrose and ethanol concentrations in process streams and effluents of sugarcane bioethanol industry by optical fiber sensor (2011) Proc. SPIE, 7753, pp. 77535ISaleh, B.E.A., Teich, M.C., Fundamentals of Photonics, pp. 193-237. , John Wiley and Sons, New York