Biodiesel is an alternative diesel fuel consisting of the alkyl monoesters of fatty acids from vegetable oils and animal fats. Biodiesel can be used in diesel engines as a pure fuel or in blends with petroleum-based diesel fuel. To maintain optimum performance and meet emission regulations, it may be necessary to measure the composition of blended fuels and adjust the fuel injection timing and other injection parameters during operation. The objective of this study was to investigate the suitability of using a commercial Flexible Fuel Composition Sensor for the detection of biodiesel composition in biodiesel/diesel fuel blends. Twelve different biodiesel fuel samples were tested including pure esters and esters from soybean oil, tallow, lard, canola oil, and yellow grease. The sensor produced a frequency output between 58.75 and 60.23 Hz for all of the biodiesel samples. Six different diesel fuel samples were also tested including commercial No. 1 diesel fuel and EPA emission certification fuel. All of the diesel fuel samples gave frequencies between 51.84 and 52.62 Hz. The frequency output of the sensor was linearly proportional to the percentage of biodiesel in the blend. The 7.14-Hz average difference from diesel fuel to biodiesel is sufficient to use this fuel composition sensor for blend detection of biodiesel blended fuels.