We report a U-bend plastic optical fiber (POF) sensor for measuring ultralow concentration of ethanol corresponding to the initial bioethanol production rate by cyanobacteria, i.e., 0.1-0.5 gL<inline-formula> <tex-math notation="LaTeX">^{\mathrm {\mathbf {-1}}} </tex-math></inline-formula> day<inline-formula> <tex-math notation="LaTeX">^{\mathrm {\mathbf {-1}}} </tex-math></inline-formula>. This production rate corresponds to 0.00499-0.0499 %wt of ethanol in total solution in terms of weight percent concentration. Refractive indices for these minute ethanol concentration values are not available in the literature hence mathematical estimation of the refractive indices for the ethanol-water solutions in this concentration range is demonstrated. The sensing principle is based on optical fiber-based evanescent wave absorption. Parameters affecting the response of an evanescent wave absorption sensor are analyzed for the intended concentration range of ethanol. Experimental results using the U-bend evanescent wave POF sensor are also presented for ethanol-water solutions having refractive indices corresponding to the bioethanol production rate. The excellent repeatability of the measurement is established and these real-time measurements show that sensor has a sensitivity of 817.76 O.D/RIU (O.D refers to optical density, unit of absorbance) with a 99.76% linearity. The limit of detection of the sensor is <inline-formula> <tex-math notation="LaTeX">9.2\times 10^{\mathrm {\mathbf {-7}}} </tex-math></inline-formula> RIU. It is also proved using refractive index calculations of ethanol-water solutions that the sensor exhibits a resolution of <inline-formula> <tex-math notation="LaTeX">10^{\mathrm {\mathbf {-7}}} </tex-math></inline-formula> RIU. The sensor of this investigation, therefore, represents a potential solution for online and real-time monitoring of the production of biofuels even at the very low-level initial concentration of product.