A novel cotton-flow sensor was designed and fabricated at Mississippi State University (MSU) in 1999. Based on that sensor, a cotton yield monitor system has been developed. Three prototypes of the MSU cotton yield monitor system were constructed. Performance of these prototypes was field-tested in Texas, Georgia, and Mississippi in the year-2000 cotton-harvesting season. About 1310 acres of cotton with different varieties and large yield variations were harvested with this yield monitor system. Operating temperatures varied from about 50 ^o F to 100 ^o F. In the Texas test, average absolute errors for the two fields were 5.9% and 5.4%, while the total errors were 2.4% and 2.5%, respectively. Results from the test in Georgia showed that the average absolute error was 5.7% and the total error was 0.9%. System reliability was tested in Mississippi by harvesting more than 1100 acres of cotton. The test indicated that the system was reliable and easy to operate. Further efforts are being made to improve the MSU cotton yield monitor system.